CHAPTER VII.
Signals—Interlocking—Block Telegraph and Electric Train Staff Instruments.
Signals.—Railway tradition alleges that on one of the early lines opened for passenger traffic, the precautions for public safety were considered to have been fulfilled by providing a man on horseback to ride along the track between the rails in the front of the locomotive engine, to give warning to persons strolling on the line, and to check the advance of the train when necessary. A very short experience of this method of working proved that the full capabilities of the locomotive could not be obtained from a restricted speed of seven or eight miles an hour, and a more comprehensive system of signalling had to be devised. By fencing in on both sides of the line, the public were prevented from making a general highway or promenade along the railway, and the problem was reduced therefore to the signalling for the trains alone.
Flags of different colours, held by flagmen stationed at suitable places, answered the purpose for a while, or so long as the authorized running speed did not prevent the train being brought to a stand after sighting a flag warning the engine-driver to stop. As speeds were increased, a longer or more distant view of signals became imperative, and tall posts, or semaphore signals, were introduced. Well-defined blades or discs placed on high posts were easily worked from the ground-level, and could be seen for long distances, thus enabling the trains to be controlled or brought to a stand before reaching the signal. The efficiency of the principle once recognized, improvements and additions were made from time to time, until we have the simple acting tall semaphore signal so universally in use at the present time. The position of the signal arms or blades in the daylight, and the colours shown by the lamps at night, form the code of signals for the proper working of the train service; and as the signal arms and lamps are both worked simultaneously by the same gearing, it
is only necessary to light the lamps to put the signals in complete condition for night-working. For some years, when the traffic was small, with trains at low speeds and at considerable intervals, one double-arm semaphore signal-post at a station was made to serve for all purposes; but as the train service became more frequent and more rapid, it was found that another semaphore or tall post signal, was necessary to give warning to the engine-driver some distance back before reaching the station or home signal. More particularly was this necessary at those stations where it was not intended that every train should stop. This new signal, called the distant signal, very soon came into general use. It was placed at distances varying from 400 to 800 yards away back from the station or home signal, and was worked by a long strained wire extending from the distant signal to a ground-lever placed near the home signal, the levers for these distant signals and home signals being thus near together and under the control of one man. More recently it was found necessary to introduce another important wire-worked signal called a starting signal, which is placed at the outgoing or departure end of the passenger platforms, lines, or station sidings, to prevent any train or engine starting or proceeding on its journey until such starting signal is lowered to indicate that the line is clear.
These simple, independent, hand-worked semaphore signals did good service for many years, but being independent and in no way physically connected with one another at junctions, or stations, or with the switches they were intended to control, it was quite possible for mistakes to arise where everything depended upon the accuracy and prompt decision of the signalman. The possibility that such mistakes could occur, and the certainty that they actually did occur, and too often with most disastrous results, led gradually to the grouping and interlocking of a large number of signal levers and switch levers together in one signal cabin. The advantages of the concentrating and interlocking of signals and switches are twofold. In the first place, one man in the signal cabin can work and control the levers for a large number of switches and signals, where formerly several men were required to be located at various places in the station-yard; but the second, and by far the most important advantage, is that with proper interlocking arrangements it is practically impossible to give conflicting signals.
With a modern interlocking frame, and assuming the normal position of all the signals to be at danger, then before a signal can be lowered for an approaching engine or train all the switches and corresponding signals, from any lines or sidings connecting with the line to be signalled clear must first be set so as to prevent any engine or train coming out of such connecting lines or switches on to the line to be made clear. In a similar manner, before the points and signals can be set to permit an engine or train to pass from a siding on to the main line all the necessary signals must first be set to danger to prevent the approach in either direction of any engine or train on the main line about to be occupied. The mechanical arrangements of the interlocking frame are so exact and complete as to effectually prevent any but the proper combination being made. An untrained or inexperienced signalman might inadvertently attempt to pull over a wrong lever, only to find it securely locked and immovable under the control of other levers. The proper sequence of levers must be made, and the accurately adjusted mechanism automatically prevents mistakes which formerly occurred with the old hand-worked signals from the oversight or confusion of the signalman.
The interlocked switches or points are worked from the signal-cabin by light wrought-iron tubing (termed rodding) or channel-shaped iron bars supported on fixed iron rollers, and the signals by galvanized wires running over light pulleys. Modern signals are always weighted at the signal-post, so that in the event of the breaking of the pulling-wire they will fly back to their normal position of danger.
The facility and precision secured by the interlocking machinery enabled other valuable accessories to be introduced for the more complete signalling and protection of train-working. Amongst these may be mentioned the facing-point bolt-lock and rocking-bar, signal-detectors at points, and throw-off or trap points.
With the old-fashioned hand-worked switches the man standing alongside could see whether the sliding-rails were properly closed, and also when the last vehicle of the train had passed over them; but when important main-line-facing switches or points are worked by rodding from a signal-cabin some distance away, it is necessary to have some reliable means to ensure that the sliding-rails are actually brought close home, and also to prevent
the switches being moved again until the entire train has passed over them. A set of switches may be carefully made and work well, but it is quite possible for some fracture or obstruction, to intervene and prevent them closing properly. If a train or engine were passing through them in a trailing direction, as indicated in [Fig. 345], the wheels would most probably force the sliding-rail home, and no disturbance would arise. If, however, the train were coming in the opposite or facing direction, the chances are that some of the wheels would take one road and some the other, and cause a derailment. The same casualty would occur if the switches were moved during the passage of the train.
To guard against the above contingencies, the facing-point bolt-lock and rocking-bar have been introduced. The system is applied in various forms, but the arrangement shown in [Fig. 480] will explain the principle generally.
A strong casting, A, is securely bolted to the top of the sleeper carrying the chairs on which rest the point ends of the sliding-rails. This casting has an internal groove or chamber formed for its entire length from C to D, as indicated by the dotted lines, and in which slides the locking-bolt B. The point ends of the switch or sliding-rails are connected by the transverse rod E, which is forged into a vertical bar form for that portion of its length, which passes through the opening, F, prepared for it in the casting A. In this vertical bar a hole or slot is cut to correspond to the exact size of the locking-bolt B, and at a distance to suit the sliding-rails when pulled over to their properly closed position. This locking-bolt, B, will not pass through the hole in the vertical bar until the sliding-rails are quite close home, and when once through the hole the sliding-rails cannot be moved until the locking-bar is withdrawn. In some cases two holes or slots are cut in the vertical bar to enable the points to be bolt-locked for both directions.
The rocking-bar is designed to prevent the withdrawal of the locking-bolt before all the vehicles have passed over the points.
This rocking-bar consists of an angle iron or tee-iron bar of a length equal to the longest wheel-base of the rolling-stock, and is carried on short pivoted arms working in cast-iron or wrought-iron brackets secured to the rails as shown in [Fig. 481]. The pivoted arms have a movement backward or forward, and when at either the one or the other extremity, the upper surface
of the rocking-bar is sufficiently below the top of the rail to be well clear of the flange of any passing wheel; but while changing from the one to the other position, and when the pivoted arms are vertical, or at half-stroke, the upper surface of the rocking-bar is about level with the top of the rail, and right in the pathway of the wheel-flange. It is evident, therefore, that when the pivoted arms are set in the forward or backward position, and one of the wheels of a train or vehicle has passed on to the rail over the rocking-bar, the latter cannot be changed or raised and pulled over to the opposite extremity so long as any one of the wheels of the train or vehicles remain over the rocking-bar.
As the same ground-crank which pulls over the pivoted arms from backward to forward also withdraws the locking-bolt B, the latter is thus held securely in the hole or slot of the transverse rod, E, until all the wheels of the train have passed off the rocking-bar. The operation of changing the points from one road to another is very simple. By means of the rodding G, worked by a lever in the signal-cabin, the locking-bolt B is first withdrawn from the slot; the points are then pulled over into the reverse position by the rodding H, and the locking-bolt B is again set back into one of the slots by the rodding G. Sometimes, for economy, the points, bolt-lock, and rocking-bar, are all three worked by one lever in the signal-cabin, and one set of rodding on the ground, as shown in [Fig. 482]; but the arrangement is neither so perfect nor so secure as that shown in [Fig. 480]. Where there are two sets of rodding and gearing, the failure or breaking of either of them prevents the complete combination being made, and indicates at once to the signalman that something is wrong; but when there is only one set of rodding a breakage may occur without giving any tangible evidence to the signalman of the defect, and he may proceed to pull over his signal lever in ignorance that the points have not been properly made and bolted. To avoid an accident taking place from the failure of either rodding or gearing, the signal-detector has been devised, so as to prevent the possibility of pulling over the signal wire until the points and locking-bar are both in their proper positions.
The signal-detector is applied in several forms; the one shown in [Figs. 480 and 483] will explain the principle on which its efficacy depends. A transverse rod, I, attached to the sliding-rail,
extends out beyond the rails, and is formed into a flat bar or plate, J, sliding through the guide-holes K, K in the casting L. Short upright levers, M and N, work on trunnions fixed in the casting, and to M and N are attached the wires leading from the signal-cabin and continuing on to the signal-posts, as shown in elevation in [Fig. 483]. Two slots are cut in the plate J to receive the curved arms of the levers M and N when they are drawn downwards to pull off the corresponding signals. Neither of the levers, M or N, can be drawn over unless there is a slot immediately under the curved arm into which it can enter. When there is solid plate under a curved arm, the short lever cannot be pulled over, and the signal therefore remains at danger. The slots in the plate J are spaced so that one will be brought into position for one of the curved arms, when the points are close home for the main line, and the other slot for the other curved arm, when the points are set for the branch line or siding. The two slots cannot be under the two curved arms at one and the same time, as one of the signals corresponds to the main line and the other to the branch line or siding.
In some forms of signal-detector the transverse rod I is joined on to a vertical bar which slides through guide-holes in a casting something similar to the arrangement shown in the casting L. Longitudinal guide-holes, parallel to the line of rails, are made in the casting a little above the transverse rod-bar, and through the longitudinal guide-holes slide two vertical bars which are attached to, and form part of, the wire connections to the two signals. The wire bars have each a small tongue or rectangular fin forged on to the under side of the bar, and there is one corresponding channel cut in the transverse rod-bar. When the switches are properly closed in one position, the channel cut of transverse bar will be opposite one of the wire bar fins, and will allow one of the signals to be pulled over, but the other wire bar cannot be moved. The closing of the switches in the reverse position moves the channel cut so as to allow the other wire bar to be pulled through, but as there is only one channel cut in the transverse bar, only one signal can be pulled over for each position of the switches.
Throw-off or trap points, are introduced to throw an engine or train off the rails of a siding on to the ballast, and so avoid a collision with any other train which may be standing or passing on the line of rails with which such siding forms a connection.
[Fig. 484] is a diagram sketch of the arrangement, in which the main-line points are indicated by the letter A, and the trap points by the letter B; one series of rodding actuated by one lever in the signal-cabin works both the main-line points and the trap points at the same time and by the same movement. The connections are so made that when the points A are set for the passage of trains on the main line, the trap points B are set open to throw off on to the ballast, as shown in [Fig. 484]; and when the main-line points A are set to allow a train to pass from the siding on to the main line, the trap points B are closed, as shown on [Fig. 485]. A disc or other signal, worked or interlocked with the points, is placed near B to notify the engine-driver when he may pass out of the siding on to the main line; but should he from any cause proceed before the points are properly set and the corresponding signal given, his engine would run off at the ends of the rails C, C, and be derailed on to the ballast. The inconvenience caused by such derailment would be trifling compared with what might result from a collision with a train standing or passing on the main line. In some cases the siding is continued onwards for a considerable distance from the trap-point rails C, C, as indicated by the dotted lines D D, and terminates with a dead end. When this arrangement can be adopted, derailment is obviated, and the engine is brought to a stand by a buffer-stop at the end of siding. On no account should trap points be placed close to the top edge of a high embankment, or up to the abutment or wing walls of an under-line bridge, where an engine running through them accidentally might fall down a considerable height, and cause serious results. All sidings joining on to main lines should be trapped as above described, and when properly signalled and interlocked in the signal-cabin, the traffic-working can be carried on with increased facility and security.
[Fig. 486] is a sketch of an average sample of an ordinary single-arm wooden signal-post, with signal-arm, lamp, spectacles, ladder, and gearing complete for wire connection to signal-cabin. When the arm stands out in the horizontal position, representing the danger or stop signal, the red spectacles will be in front of the lamps, and will show a red light to an approaching train. When the arm is lowered, as indicated by the dotted lines, the second spectacle will be in front of the lamps, and will show either a white or green light (according to the accepted code) as
an all-right signal for the train to proceed. For many years a white light was adopted for the all-right signal, but latterly, to prevent confusion with other white lights about a station, there has been an increasing disposition to use a green light as an all-right signal. Several railway companies have already effected the change, and others have arranged to follow their example. The counter-weight W keeps the signal-arm to the danger position, except when it is raised by the pulling over of the signal-wire from the signal-cabin working over the pulley P. Should the wire break when being pulled, the weight W falls down to the stop-plate, and the signal-arm rises to danger. The signal-posts may be of wood, wrought-iron, steel lattice-work, or cast-iron.
The arms of distant signals should be cut to a fish-tail shape, as in [Fig. 487], to distinguish them from other signals. Goods-line signals should have a thin sheet-iron ring, as in [Fig. 488]. Sometimes purple glass is used instead of red glass for the spectacles of goods signals. Letters or numbers may be attached to signal-arms to signify the lines or sidings to which they correspond. Special signals are sometimes made with the arm working on a centre pin, as in [Fig. 489].
At junctions or places where two or three signals have to be fixed near together, it is customary to carry them on a bracket signal-post, as in [Figs. 490 and 491]. The former represents the home signals at an ordinary junction, the taller signal being for the main line and the lower one for the branch line. [Fig. 491] shows the home signals at a junction where there is one line turning out of the main line to the left and another to the right. The taller signal in this case also serves for the main line and the two lower signals for the branch lines.
In important station-yards, where there are a large number of lines and sidings running side by side, it is not always convenient or possible to place the respective signal-posts in suitable positions between the lines. To overcome the difficulty, the signals are erected on light overhead lattice girders, as shown in [Fig. 492]. In some cases, for want of a better position, or to obtain a more comprehensive view of the lines and signals, the signal-cabin is built on lattice girders, as in [Fig. 493].
Ground or disc signals are fixed at the ground-level, and are worked in conjunction with trap points or outlet switches from sidings. In some cases they are worked direct by a connecting-rod
from the switches, and serve merely as indicators to show whether the switches are lying for or against an engine passing out of the siding. In other cases they are worked independently from the signal-cabin by a separate lever and wire connection, the interlocking being so arranged that the lever working the switches must be pulled over before the lever working the disc signal can be moved. In one type the disc signal is fixed to a short vertical axis, as shown in [Fig. 494], and by means of a cranked arm is made to rotate a quarter of a circle, so as to exhibit either a stop or advance signal according to the position in which the switches are lying. In another type, the lamp is fixed, and the red disc, with a red glass in the centre, is made to assume a horizontal or vertical position by a rod and crank, as shown in [Fig. 495].
A simple arrangement of rodding and rollers for switch connections is shown in [Fig. 496], the number of sets of rodding being determined by the number of connections to be made. [Fig. 497] is a rodding compensator, to compensate or adjust for the difference in length of the rodding arising from variations in the temperature. The compensator may be used either vertically or horizontally, according to space or circumstances.
Strong wrought-iron or steel cranks of different angles will be required when changing the direction of the rodding, or connecting to switches and facing point-locks. They must be firmly secured to strong timber framework well bedded in the ballast. For cranks working switches and bolt-locks, it is better to use extra long timbers under the rails instead of the ordinary sleepers. Cross-pieces can be bolted to the ends of the long timbers, and the cranks placed practically on the same timbers carrying the permanent way. By this means the rails and cranks can always be maintained in their proper relative positions as to distance, line, and level.
Without a large series of diagrams it would be impossible to adequately describe the extent of signalling and interlocking required at large terminal stations and important roadside stations, but one or two simple examples may serve to illustrate the general principles.
[Fig. 498] represents the modern grouping of signals considered necessary at an ordinary double-line junction, showing all the signals at their normal or danger position. The numbers marked on each indicate the numbers of the levers in the interlocking
frame of the signal-cabin. Four distinct sets of trains have to be dealt with at this class of junction, and the interlocking must be so arranged that when the signals are lowered for the advance of any one train, no conflicting signals can be given to any other train.
Assuming a train approaching from A, which has to continue on the main line past B on towards C, then the levers in the signal-cabin must first be pulled over to set the points 9 and bolt-lock 8 in proper position for the main line; and this operation will release the levers which have to be pulled over to lower the signals 5, 4, and 6, but at the same time will lock, and prevent the pulling over of the levers or lowering of the signals 2 and 1 for a train from A to B and D, or of the signals 14 and 15 for a train from D to B and A. The levers will, however, be free to pull over for setting the points 12 and lowering the signals 16, 17, and 13 for a train on the main line from C to B and A.
In a similar manner, assuming a train approaching from D, which has to continue up to the main line at B and on towards A, then the lever in the cabin must first be pulled over to set the trailing points 12 in proper position; and this operation will release the levers which have to be pulled over to lower the signals 14 and 15, but at the same time will lock, and prevent the pulling over of the levers or lowering of the signals 5 and 4 for a train from A to B and C, or of the signals 16 and 17 for a train from C to B and A. The levers will, however, be free to pull over for setting the points 9 and bolt-lock 8 and lowering the signals 2 and 1 for the passage of a train on to the branch line from A to B and D.
For a train from C to B and A, the levers 12, 16, 17, and 13 would be required, and these would lock levers 14 and 15, and prevent the approach of any train from D to B, but they would leave free the levers necessary either for a train from A to B and C, or for a train from A to B and D, but only one of them at a time, the setting of the one series locking the other series.
A train from A to B and D would require the proper setting of the points 9, bolt-lock 8, and signals 2, 1, and 3; and these would lock 5 and 4, but would leave free the levers necessary either for a train from C to B and A, or for a train from D to B and A, but only one of them at a time.
The cross-over road from the UP to DOWN main line, near the
letter B on sketch, is only intended for use in case of break-down or accidents, and the normal position of the points is to lie clear for the passage of trains on the main lines. To use the cross-over road, the whole of the signals must first be set to danger before the points 7 and 7 can be opened to permit the passage of an engine or train from the one main line to the other.
The starting signals 6 and 3 should be placed sufficiently far away that the longest passenger or goods train may stand between them and the clearance points at G and E. These starting signals are of great service to train-working at junctions. Supposing a main-line train from A arriving at B before the section from B to C was clear, such train could be brought to a stand at signal 6, and remain there while another train from A was allowed to pass B, and proceed onwards towards D; or a branch-line train from A to D could be brought to a stand at 3, to allow a main-line train to proceed onwards from A to B and C. The starting signal 13 should be placed well in advance of the cross-over road to control anything passing from one line to the other.
[Fig. 499] shows the modern grouping of signals for an ordinary single-line junction. The arrangement is almost practically the same as for the double-line junction shown in [Fig. 498], there being the same four distinct sets of trains to be controlled, but not any cross-over road. The signal-cabin is placed on the main line, a little in advance of the facing points, and a well-fenced-in gangway, the same height as the engine footplate, is carried out the proper distance from the rails, on which the signalman can stand to hand over or receive the train staff from the engine-driver when passing.
At stations and places where there are several sidings and lines connecting with the main lines, at considerable distances apart, it will be necessary to have two or more signal-cabins placed in suitable positions, not only for expediting the working of the constant shunting movements, but also to insure that there is a signal-cabin within the regulation distance of all facing points on the main line. So far as the main line is concerned, the interlocking of these cabins must be connected, the one with the other, by slotting, or co-acting gearing, in such manner that the cabin in advance shall always be able to control the cabin in the rear in the lowering of the main-line signals for an approaching train. [Fig. 376] is a diagram sketch
of a typical double-line roadside station with two signal-cabins. The NORTH cabin has to work the signals and points in connection with the goods-shed, goods-sidings, and market branch, and the SOUTH cabin, those in connection with the coal and cattle sidings; and each of the cabins to work the signals and points of that portion of the main line adjoining its own cabin. For siding working, each cabin is quite distinct and independent of the other, but for main-line working the lowering of the signals can only be effected by the joint operation or co-acting of both cabins.
Assuming a train approaching from A to proceed in the direction towards B, then, before the signalman in the NORTH cabin can lower the UP home-signal C, the signalman in the SOUTH cabin must first pull over his lever and release the slot which retains the signal C at danger, and in doing so the levers in his own cabin will stand locked, and prevent the lowering of the signal D, or opening of points E to allow access from the sidings to UP main line. The cross-over road F G will also be locked for main line clear. When the slot has been released from signal C, the signalman in NORTH cabin can lower the UP home signal C, but before he can pull over the lever for this purpose he must first lock the points H, to prevent access from the sidings to the UP main line, and also the points K L of the cross-over road, to keep the main line clear. A similar operation has to be gone through for a train approaching from B to proceed in the direction towards A, when the signalman in NORTH cabin must first withdraw the slot from the DOWN home-signal M before the signalman in the SOUTH cabin can lower that signal. A small automatic disc is placed in the cabin to indicate to the signalman when the slot has been withdrawn by his colleague in the neighbouring cabin, and for facility of working, the two cabins are usually placed in communication with each other by telegraph or telephone.
At some stations similar to the above, where there is a very frequent train service, with several of the trains running through without stopping, it is the practice to have a second or lower arm to the home signals C and M, as shown on the diagram, these lower arms being only pulled off for through or non-stopping trains, as an indication to the engine-driver that the line is clear in the section ahead.
In addition to the leading signals shown in the sketches,
there are shunting signals for the movement and marshalling of trains—setting-back signals in connection with the making up of passenger trains; taking on or off passenger carriages; or moving out empty passenger carriages; and many other special signals which become necessary for the working of a large and complicated train service.
The above simple diagrams will explain some of the principal requirements to be kept in view when working out signalling arrangements. Where the lines and sidings are very numerous, as at important junctions and large terminal stations, the signalling becomes very intricate, and may require three or four cabins, slotted together in such manner that the necessary co-acting may be insured for the proper controlling of the mainline signals. Many of these signal-cabins contain a large number of levers, some of them having as many as a hundred, and a few of them two hundred and forty levers, or more, all of them so carefully arranged that no conflicting signal can be given. Not only has much skill to be exercised in the accurate adjustment of the interlocking machinery, but much study must be devoted to determine the exact duty of every lever, for the locking or releasing of other levers.
Signal-cabins may be built of stone, brick, or wood. They should be roomy, well ventilated, and have abundance of light. Every signal-cabin should be placed in the position from which the signalman can obtain the best view of the signals and points under his charge. The height of the cabin floor will depend upon any obstacles that may intervene between the cabin and the signals, such as over-line bridges, station roofs, buildings, or other obstructions. Sometimes the floor has to be kept as low as five feet above rail-level, to secure a line of sight under the over-line bridges; and in others the floor has to be raised twenty, or even thirty feet above rail-level.
[Figs. 500, 501, and 502] show plan, transverse section, and elevation of a signal-cabin suitable for a small roadside station. The lower story and chimney-stack are of brick, and the upper story of wood, with slated roof. There is room for an interlocking frame of twenty or twenty-five levers, and space at the end of the cabin for the block-telegraph instruments, or electric train-staff instruments. The roof-work is open up to the slateboards, to obtain as much air capacity as possible. In the transverse section a winch for working mechanical gates is
shown at the end of the interlocking frame. There is a liberal amount of glass, and two or three sliding windows, which the signalman can open to enable him to speak to the engine-drivers or others during shunting operations. The lower story of the cabin can be utilized for trimming lamps and keeping a small supply of coals and other stores. When working after dark the lamps in the cabins should be well protected by shades, to prevent the lights being seen by engine-drivers, and mistaken for signals.
Interlocking.—There are several systems of interlocking, each of them varying considerably in the form and mode of application, but all of them having the same general object of securing or releasing the necessary levers for each combination of signalling movements. A brief description of one of the systems will explain the order in which the movements have to be made, and the security which can be obtained by the locking.
[Figs. 503, 504, and 505], are sketches illustrating one of the types of wedge and tappet interlocking. Each lever works on a fulcrum or pinion as at A, and has a lower arm B for lifting the rods leading off to points or signals, and an arm C to carry a counterweight when necessary. Cast-iron braces D are placed at convenient distances between the series of levers to carry the top frame E on which the lever floor casing F is bolted. This casing is continuous from end to end of the locking frame, with the exception of the narrow openings through which the levers travel when moving backwards or forwards. The sleeve-block G, resting in the depressed portions of the arc, retains the lever in position. When taking hold of the main lever L, the signalman’s hand draws the small side lever M, close to the main lever, and raises the sleeve-block G sufficiently high to pass over the top of arc F, the lever L can then be pulled or pushed over, and the block G will fall into the depression at the end of the stroke when the hand is removed. N is a tappet or thin flat bar attached to the main lever, and which works backwards or forwards between the wedges in the wedge frame O. The wedges move horizontally between guide pieces, and work either singly or are connected by the lower slide bars to other wedges some distance away on the frame according to the position of the levers which have to stand or move in unison for the releasing or locking. A strong cover is placed over the wedge frame to keep out the dirt.
[Figs. 504 and 505] show plan views of four levers in a signal cabin taken just above the level of the tappets. In [Fig. 504], all the levers are in their normal or forward position, with the home and distant signals at danger, and the facing points leading into loop or siding lying for main line. Previous to the approach of a train on the main line, the home and distant signals have to be lowered, and will require the pulling over of levers 1 and 2; but these levers cannot of themselves be moved, as the wedges P and Q are locked by the straight side of lever 3. The operation would therefore be as follows:—points lever 4 being set in its normal position for the main line would remain forward, lever 3 working the facing point bolt-lock would be pulled over, and in doing so would move the wedge R to the right into the recess of tappet of lever 4, locking that lever, and presenting the recess of its own tappet ready to receive the wedge Q. Lever 2 can then be pulled over, and will move the wedge Q to the right into the recess of tappet of lever 3, and present its own recess for wedge P. The pulling over of lever 1 completes the series, by moving the wedge P over to the right into the recess of tappet of lever 2. [Fig. 505] shows the positions of the tappets and wedges with the levers 1, 2, and 3, pulled over to make the combination described. Upon examination, it will be seen that levers 2, 3, and 4, are all securely locked, the points cannot be moved, nor the facing point bolt-lock withdrawn, nor the home signal changed until the lever 1 is pushed over again into its normal or danger position. To restore the levers to their forward position, they must be set back in the reverse order to which they were pulled over. To simplify the explanation, only four levers are shown in the above sketches, but the principle is constantly extended out to a very large number of levers, and in many cases necessitates the introduction of several rows of wedges as indicated by the dotted lines. In some instances a combination is effected by pulling a certain lever only half over. In some systems the preliminary action or spring handle locking is adopted, in which the locking is actuated by the small side lever, similar to the one marked M on [Fig. 503]. The advocates of this arrangement claim increased security and precision in the interlocking, while on the other hand it is alleged that the mechanism is rendered more complicated without any corresponding advantage.
Detached Lock.—Sometimes there is in the vicinity of a
railway station, a siding which is too far away to be worked direct from a signal cabin, and not sufficiently used to warrant a separate cabin. Such sidings can be worked by a small ground frame opened or locked by a special key attached to the interlocking machinery in the adjoining signal cabin on a double-line railway, or attached to the train staff on a single line.
[Fig. 506] shows the arrangement applied to a double line with the outlying siding turning out of the UP main line, the points lying in a trailing direction for the running trains. Before the special and only key can be withdrawn from its seat in the interlocking frame of the signal cabin, all the UP main line signals must be set to danger, and cannot be moved from danger until the key is restored to its proper seat again. When the key is removed from the signal cabin, it can be taken to the ground frame at A, inserted in the key opening, and by turning it partly round, will release the bar which locks the levers of the facing point bolt-lock and the points. When these two levers are free the points can be opened, and vehicles moved into or out of the siding B C, but the special key cannot be withdrawn from the ground-frame A, until the points and facing point bolt-lock are put back again into their normal position for main line working. When the operations at the siding are completed, the special key can be removed, and taken back to its proper place in the signal-cabin, and ordinary working be resumed.
[Fig. 507] shows the application of the detached locks on a single line, and is a sketch of a portion of railway on which there is a small station B, with a goods siding F G, where the traffic is too small to require anything more than ground frames and detached locks. An engine-driver before leaving the station A, receives a train staff, which gives him possession of the line as far as C, including of course the intermediate station B, and this staff he must carry with him and hand over to the signalman on his arrival at the end of the section at C. At each of the points D and E is placed a two lever ground frame, similar to the one shown in [Fig. 506], and attached to the train staff is a key, which will operate either of the two ground frames, but only one at a time, as the key must be inserted before the levers can be moved. When the train is proceeding in the direction from A to C, it will be more convenient to shunt vehicles into or out of the siding F G, by means of the points E, but when proceeding from C to A, the points D will be more convenient.
Whichever of the points be used, they must be set, and bolt locked for the main line before the train staff and its key can be withdrawn from the ground frame and restored to the engine-driver. As the siding is trapped at F and G, it is impossible for any vehicles to be moved out on to the main line except through the medium of the train staff and key. The same arrangement of detached lock is equally available for a single siding with only one set of points.
Electric Repeater.—It will sometimes occur that on account of a curve or other obstacle, the arms and back lights of a distant or other signal cannot be seen from the signal cabin, and it is necessary to introduce an electric repeater. This little instrument consists of a miniature semaphore signal fixed in a metallic box with a glass front, and placed on a stand about a foot above the floor level immediately in front of the signal lever for which it is intended to serve as an indicator. Like the signal proper, the normal position of the miniature semaphore is at danger, but when the signal lever is pulled over in the cabin, the rod that pulls down the arm on the signal post effects a contact with an electric circuit which lowers the arm of the miniature semaphore at the same moment that the signal arm proper is lowered, and gives visible indication in the cabin that the signal is working. [Fig. 508] is a sketch of one form of electric repeater.
Detonators or fog signals are largely used in foggy weather and snowstorms, when the out-door signals cannot be seen from an approaching train. At such times the atmosphere is so dense, and the surrounding objects so obscured, that an engine-driver is totally unable to distinguish the usual landmarks which guide him on the approach to a station or semaphore, and he might easily pass by a signal unless he received an audible signal to indicate the position of the one that is invisible. Detonators are usually made in the form of a circular tin or metallic case about two inches in diameter, and three eighths of an inch thick, with soft metal clips on opposite sides for bending over and securing to the rails. The case is filled with detonating powder, which is crushed by the first wheel passing over it, and explodes with a loud report. It is customary to use these detonators in pairs placed a short distance apart in case one of them should fail to explode.
Fog-signalling regulations vary on different railways, but
the arrangements are generally carried out somewhat in the following manner. During the prevalence of a fog or snowstorm, a fog [signalman] is placed near each of the signal-posts to be protected, and is supplied with a hand signal-lamp, hand-flags, and a packet of detonators. So long as the arm of the signal-post at which he is alongside stands at danger, he must keep two detonators on the rail of that line which the signal controls, and also show a RED hand-signal (hand-flag by day, and hand-lamp after dark) to the approaching train. When the signal arm is lowered to show that the line is clear for the passage of the train, the fog signalman must remove the two detonators, and show a GREEN hand-signal (flag, or lamp) to the approaching train. When an engine driver hears the report of a detonator crushed by his engine, it is his duty to shut off steam immediately, and bring his engine to a stand, after which he must proceed very cautiously, until he receives further signals by hand or otherwise, or receives the line-clear signal to continue on his journey. Detonators are also of great service both in fine or bad weather, in cases of a wash away, a failure of works, or obstruction on the line, when a hand-signal may not be seen, but a detonator must be heard.
Mechanical Gates.—Mechanical gates, worked and controlled from the inside of a signal-cabin, are now very largely adopted for public road level-crossings instead of ordinary hand-gates, opened and closed by a gateman walking from side to side of the line across the rails. Being worked from inside the cabin, they remove all possibility of the gateman being struck by a passing train; they move simultaneously, and can be opened or closed in very much less time than hand-worked gates, which have to be moved one by one, and being interlocked with the signals, the mechanical gates cannot be placed across the lines of rails until the train-signals in each direction are set at danger. When set for either train traffic or public road traffic, the gates are held firmly in position by metal stops, rising out of cast-iron boxes lying flush with the ground, and worked by a separate lever in the signal-cabin.
Assuming the gates to be set for train traffic, and it is desired to open them for the public road traffic, the first operation will be to pull over the levers, and raise the signals in each direction to danger, and thus release the stop-lever, which can then be pulled over, to lower the gate-stops and allow the gate-winch to be turned, and the gates moved round into correct
position. The stop-lever must then be set back to raise the stops and hold the gates secure. The train-signals will be retained at danger by the interlocking gearing, and cannot be lowered until the gates are set back again across the public road, and the gate-stops raised.
It is frequently urged that the celerity with which mechanical gates can be swung round and closed across the public road, is in itself a source of danger, and that persons preparing to cross the line might be struck by a moving gate, unless they received a distinct warning that such closing was about to take place. There is no doubt persons have been struck by such gates when closing across the road, and heavy claims for injuries have been decreed against railway companies, who were unable to prove that the man in charge had called out or given warning before moving the gates. To ensure that due and undeniable warning shall always be given, a firm of signal-makers have patented an appliance by which a powerful electric gong, fixed on the top of a tall post close to the gates, is sounded automatically by the gate machinery itself, and before the gates actually commence to move. As previously described, the pulling over of the lever to lower the gate-stops is the first operation to be performed whenever it is necessary to change the position of the gates, and it is the pulling over of this lever which actuates the apparatus, by bringing two electric points into contact, and thus starting the ringing of the gong or alarm. The gong continues to sound until the gates are moved over, the gate-stops raised, and the stop-lever put forward again into its normal position. The arrangement is very simple and very effective, and being purely automatic must work as regularly as the stop-lever. The tone and volume of the gong can be varied to suit circumstances. The public soon become familiar with its sound, and recognize its meaning.
[Figs. 509 and 510] give sketch plan and elevation of a set of mechanical gates for a public road level crossing on a double line of railway. The signal-cabin should be placed within a few yards of the gates, to enable the man in charge to have a good view of the persons and vehicles passing over the roadway. The underground gearing for working the gates and stops, must be protected by iron or wooden casing. The swinging portion of the wicket gates is closed, and held by a separate lever. The gates shown on the sketch are for a crossing on the square, but
they can be equally well arranged for an oblique crossing, and of widths to suit the locality.
Block-Telegraph Signalling.—However complete the outdoor signals and interlocking at any station, they can only control the movement of trains within their range, and something more is requisite to ensure the safe working of the traffic over the long lengths of line between stations. For some years a time-interval was allowed for the working of trains following one another on the UP and DOWN lines of a double line railway, no train being allowed to leave a station sooner than a fixed number of minutes after a previous train had started in the same direction. With this system there was always the risk that the first train might be overtaken and ran into by the second, and especially in the night time, or when the atmosphere was at all foggy. The electric telegraph was then called in to assist in the train-working, and brief telegrams were passed between the stations announcing the departure and arrival of trains. The increased security and convenience thus obtained led to the introduction of special electric telegraph instruments, devoted to the exclusive duty of train-working. These instruments, termed block telegraph instruments, are now almost universally used on all double lines of railway, and have largely contributed to the safe and efficient working of an ever increasing traffic. They are made in various forms, but the object of each is to ensure that before any train is allowed to start from, or pass any station, the signalman at that station shall receive from the signalman in the cabin in advance a distinct visible signal that the line is clear, and free of any train up to the cabin in advance; and also that after the train has been despatched, the signalman in the rear shall be at once advised when the train has arrived at the signal-cabin in advance. [Fig. 511] is a sketch of one type of block-telegraph instrument, in which the leading feature is the miniature signal-post with its two arms, an arrangement which readily appeals to the eye of the signalman as being so similar in form and action to the fixed signals in the station. Each instrument is supplied with a bell or gong, by which the adjacent signalmen can communicate with each other, in accordance with a fixed code of signals which defines the relative numbers of strokes of the bell or gong, to represent certain regulation calls and answers. In the signal-cabins of the intermediate stations, two
block-telegraph instruments are required, one for the section of the line to the left hand of the cabin, and the other for the section to the right. At the terminal stations only one instrument is required.
In the instrument shown in [Fig. 511], the upper arm of the miniature signal-post is coloured RED, and is moved by electricity through the medium of the block telegraph instrument in the signal-cabin in advance; and until this RED signal be lowered to the line clear position by the signalman in the cabin in advance, no train must be allowed to start from or pass the cabin in the rear. The lower signal-arm coloured WHITE is lowered by the plunger A on its own instrument by the signalman in charge, and at the same moment lowers by electricity the upper or RED arm of the block-telegraph instrument in the signal-cabin at the other end of the section. The lower or WHITE arm is thus restricted to the signals sent away from the signal-cabin, while the upper or RED arm is restricted to signals received in the signal-cabin. In the centre there is a round handle B, which rotates a circular disc inside the instrument, and on this disc are painted three distinct train inscriptions, only one of which can be seen at a time through the glazed opening. One inscription has the words ALL CLEAR painted in black letters on a WHITE ground; another has the words TRAIN ON LINE painted in white letters on a RED ground; and the third has the words TRAIN OFF, BUT SECTION BLOCKED painted in black letters on a GREEN ground. The instrument is considered to be in its normal position when the GREEN inscription is in view, and both the miniature signal-arms raised to danger.
[Fig. 512] represents a portion of double line divided out into sections, or working blocks, between the stations B, C, and D. Each station is provided with the necessary block-telegraph instruments, and the usual distant, home, and starting semaphore signals.
[Fig. 513] is a diagram sketch showing the pair of instruments as they stand on the instrument-tables in the signal-cabins B and C, where B2 and C1 are the instruments which work together for the block section BC. Supposing a DOWN train proceeding from A in the direction of F, and approaching the signal-cabin of the block station at B, the DOWN starting signal standing at danger; then by the code of signals on the bell or gong the signalman at cabin B would communicate with the signalman
at cabin C, to obtain line clear, so as to allow the approaching train to proceed on to C. If the previous train in the same direction had already passed C, and there was not any obstruction on the line, the signalman at C would give line clear for the DOWN train, and to do so he would turn his circular disc to show the WHITE inscription ALL CLEAR, and then push in the plunger of his C1 instrument, lowering the DOWN or white arm, K, of his own instrument to the position shown by the dotted lines, which operation would at the same moment lower by electricity the DOWN or red arm, G, of the instrument B2 in cabin B to the position of the dotted lines. The signalman at B would then lower his starting signal, to allow the DOWN train to proceed on towards C, and immediately the train had passed the starting signal he would, by means of his bell or gong advise the signalman at C that the train had entered the section, or block BC, and the signalman at C would at once turn his circular disc to show the RED inscription TRAIN ON LINE, and use his plunger to raise to danger the DOWN or white arm, K, of his own instrument, and at the same time raise by electricity the DOWN or red arm, G, to danger in the instrument B2 in cabin B. The section BC would then remain blocked until the DOWN train had arrived, or passed the station C, when the signalman there would, by means of his bell or gong advise the signalman at B that the DOWN train had passed out of the section, and would turn his circular disc to show the GREEN inscription TRAIN OFF, BUT SECTION BLOCKED. Both instruments would then be in their normal positions, with the arms raised to danger, and ready for further train operations. In a similar manner for the UP-line trains on the section or block between C and B, the signalman in B cabin would turn his circular disc, and use his plunger to lower the UP or white arm, H, in his own instrument, B2, and at the same moment lower by electricity the UP or red arm, I, of the instrument C1 in cabin C, the other operation for train on line and train off being carried out for the UP train in the same routine as for the DOWN train. The outdoor fixed signals, or distant home and starting semaphore signals, have all to be worked to correspond to the block telegraph signals, and as the latter are always received well in advance of an approaching train, it follows that when the line is clear, the outdoor signals can be lowered so as to allow a through or non-stopping train to pass a block-telegraph station at full speed.
Where the traffic is moderate, it may be sufficient to have block-telegraph instruments at each of the stations, but with a very frequent train service it will be found necessary to divide the line into shorter sections, and erect signal-cabins and block-telegraph instruments at intermediate points between stations.
The code of bell or gong signals is extended to include various matters in connection with the train-working. For example, when a DOWN train is passing cabin B at full speed, the signalman may observe that there is something wrong—a carriage or waggon on fire, a tail-lamp missing, or other irregularity. It is too late to stop the train with his own signals, but by means of his bell or gong he can call upon the signalman in cabin C to stop and examine the train, and the DOWN distant and home signals at C can be raised to danger before the train reaches the cabin at C.
In every block-telegraph signal-cabin there is a train-book in which the signalman has to write down the time and description of every arriving or passing train, and, as this book lies before him, he has a complete record of the train-working, with the particulars of the exact times when the line clear signals were given, and also when the train arrived or passed his signal-cabin.
To guard against the possibility of a signalman inadvertently giving line clear, or allowing another train to pass his cabin before the previous train had reached the signal-cabin in advance, some railways have adopted the lock and block system. By this arrangement the starting signal at any cabin is electrically and mechanically locked from the cabin in advance, and can only be released or lowered by the action of the outgoing train itself when passing over a treadle or other appliance connected with the rails of the running-line at the signal-cabin in advance. This method practically gives the train the complete control of the section; and any signalman attempting, in error, to lower his starting signal would find it to remain fixed to danger and immovable, until released by the arrival of the train at the advance cabin.
Train-staff for Single Line.—When there is only a single line of railway for both an UP and DOWN train-service, very definite precautions must be adopted to prevent the meeting or collision of trains travelling in opposite directions. Where the piece of
single line is short, and can be worked by one engine in steam, or two coupled together, no collision can take place, as the train-service will be limited to the one train moving backwards and forwards over the section; but with a long length of single line, including a large number of stations, necessitating several trains, some clear and comprehensive regulations must be introduced. For a long time the simple train-staff was found to give the desired security; there was only one staff for each pair of adjoining staff-stations, and no train was authorized to run without the staff, and as the staff could only be on one train at a time, the precaution against collisions was looked upon as complete. These staffs, which were generally made of brass, or other metal, were sufficiently large to be conspicuous when placed in the stand prepared for them on the engine. They were lettered to correspond to the stations to which they belonged, and were made in different patterns to distinguish them for their respective sections. No train was allowed to start from a station until the engine-driver received from the station-master the proper staff to authorize him to proceed to the next station, and on his arrival there it was the duty of the engine-driver to hand over the train-staff to the stationmaster of that place, and wait for another train-staff to authorize him to proceed over the next section. So long as the train service could be evenly arranged, and that there was always an UP train to take back a train-staff which has been carried out by a DOWN train, the simple staff worked most efficiently; but as the traffic increased, and two or more trains had to be despatched in the DOWN direction before one had to run in the UP direction, some auxiliary arrangement had to be introduced. This was effected by issuing train tickets, kept in a locked-up box, which could only be opened by the key attached to the train-staff. A properly dated train-ticket was handed to the engine-driver of the first DOWN train, and, if necessary, a second train-ticket to the engine-driver of a second DOWN train, and then the train-staff itself was handed to the engine-driver of the third DOWN train. There were one or two serious drawbacks to this train-staff and ticket-working. As there was only a time interval between the starting of the trains, the one train might overtake and run into the other with disastrous results. Again, a second or third train, which was put down in the schedule, might be withdrawn at the last moment, and the staff left behind at a station when it
was required at the opposite end of the section, thus causing much confusion and delay. The ordinary electric telegraph could have been utilized to assist in regulating these train movements, but it was felt that a mere telegraph message was not sufficient to ensure positive safety, and that something more tangible was required in the shape of a staff, or token, without which no train should be allowed to travel on a single line of railway. To meet this requirement, the electric train-tablet, and the electric train-staff instruments have been invented, each of them being so arranged that upon any one section, or pair of instruments, a tablet or train-staff may be taken out from the instrument at either end of the section, but when once taken out, no other tablet or train-staff can be withdrawn from either instrument until the first has been delivered and placed again in one or other of the two instruments.
[Figs. 514, 515, and 516] are sketches of an electric train-staff instrument which has been very largely adopted on single lines, both at home and abroad.
In a similar manner to the block-telegraph instruments for double line, the electric train-staff instruments have each a bell or gong by which the adjacent signalmen can communicate their calls and answers in accordance with a regulation code. In the signal-cabins of the intermediate stations two instruments are required, one for the staffs belonging to the section to the left of the cabin, and the other for the staffs of the section to the right. At a terminal station only one instrument is required.
The head of the instrument contains the electrical and mechanical locking apparatus which controls the withdrawal of a train-staff, or is acted upon by its insertion. The circular name-plates and pointers, together with the galvanometer in the centre, serve as indicators to guide the signalmen in carrying out the various operations. The staffs usually consist of thin steel tubes, solid at the ends, with metal rings fixed upon them, as shown in the sketch, the number and position of the rings varying according to the section or pair of staff stations to which they belong; this difference in the rings effectually preventing the possibility of one set of staffs being used or inserted in either of the instruments of the adjoining sections. The staffs rest normally in the long vertical slot A, with the rings fitting in vertical grooves, which prevent the removal of any staff except by passing it along the curved slot BC, and out by the
opening D, of large diameter. The electrical and mechanical locking apparatus is placed at the curved slot, and until the locking-bolt, which stands across the passage of the curved slot, be lifted by the joint operations of the signalmen and their instruments at both ends of the section, no staff can be withdrawn. When the instruments are standing in their normal position of “staffs in,” the signalmen can arrange between them to withdraw a staff—say either from the NORTH cabin instrument or from the SOUTH cabin instrument of the section, but only from one of them; and the act of taking out that staff automatically locks both instruments, and prevents the possibility of taking out any other staff from either instrument until the staff already removed is restored and inserted in one or other of the instruments. From the above description it will be seen that the electric train-staff instrument provides for the safe working of two or more trains proceeding, one at a time, in the same direction over a section of single line, each one being supplied with a train-staff, which must be handed over at the end of a section before another staff can be issued for a following train. Should the train-staffs accumulate in one instrument, in consequence of more trains running in one direction than another, a re-distribution of staffs is effected by the authorized persons according to fixed regulations.
In the diagram sketch, [Fig. 517], a piece of single line is shown divided into sections or blocks, with loops or passing-places at the stations. At the station E a train-staff taken out of the instrument F serves for the section up to the instrument L at the station H; and on the train-staff is a key which will open the detached locks on the points of the small intermediate station, G, as described in [Fig. 507], in connection with the working of detached locks. At the station H the engine-driver receives another staff from the instrument M, which takes him to the instrument N at station K, and in like manner on this staff is a key which will open the detached lock on the colliery siding points at I. At stations H and K are shown loops, or short pieces of double line, with platform to enable an UP train to cross or pass a DOWN train. The distance apart of the electric train-staff stations will depend greatly upon the number of the trains, and for a frequent train-service it may be necessary to have the instruments at every station, whether large or small. The electric train-staff is of great advantage in the working of
ballast or construction trains, as a staff may be taken out of the instrument F at station E, which will give possession of the section as far as station H, and when the ballasting operations—which may be very near to E—are completed, the train can return to E, and deliver the staff again to the instrument F, instead of having to run the entire distance to station H. Although carrying a train-staff, the engine-driver must approach stations cautiously, and obey the fixed signals in the usual manner.