train and enter the section in advance. The advanced starting signal is usually placed about 300 yards from the cabin, in front of the starting signal, and enables a train which has passed the platform starting signal for the purpose of picking up waggons from sidings, or to clear the section in the rear, to be brought to a stand without entering the section in advance. It may be explained that all semaphore signals are fixed in such a way that on approaching them the arm appears on the left-hand side of the post, and this arrangement enables the same post sometimes to be used for carrying both "up" and "down" signals.
The disc signal is used to indicate to a driver whose train is in a goods siding which joins the main line when he may leave the siding and pass out on to the main line. It is really a disc of metal, with a lamp, carried on a spindle fixed near the ground, and which turns on its axis. The position of the disc by day and the colour exhibited by the lamp at night furnish the required indications as to the state of the line. Fig. 10 shows a simple arrangement of signals at a station, and Fig. 11 shows the ordinary signals required at a simple junction where there is no complication of lines.
At a small roadside station, where there are no sidings connecting with the main line, the signals are easily worked by detached levers, placed in any convenient position; but at larger stations, where there are many signals and points to be actuated, a means of concentrating the working becomes necessary, and this is done by means of the "signal cabin," which brings us to the question of interlocking. The signal cabin contains a most complicated piece of mechanism, called the "locking
