Railway Construction

by William Hemingway Mills

[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.