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.