[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.