SIGNALLING Semaphore and Light Signals.—The signalling system is the means adopted to control the movements of a train by means of visible signals to the crew of the train. The signal may consist of an arm or other movement, replaced by lamp signals at night, and in the usually accepted sense of the term applies primarily to the signals fixed along the track. These fixed signals normally consist of discs, or semaphores, mounted on posts; the varying movements to indicate the two aspects of "danger" or "line clear" being re placed at night by a coloured light, red being standard for danger in Great Britain and green for "line clear." The improvement in electric lighting permits these red and green lights, known as "colour light" signals, to be seen clearly in daytime, and their use is increasing in preference to the semaphore and disc signs. Many early British signals were of the disc design, being placed at each station and road crossing gate. Space between trains was obtained by enforcing a time interval, often ten minutes, between the trains. The "semaphore arm" was used as early as 1845, while the use of more than one arm at junctions gave the engine driver a route indication. It has since become customary in Great Britain to place the semaphore signal for the less important lines on a bracket post, usually "stepped" so as to give the more important line the higher position.
With increasing speed it became necessary to give advance warning of a "stop" signal at danger; this signal in advance of the stop signal has come to be known as the "distant" signal, and being a warning only is fitted with a "fish tail" and shows a "yellow" light for the danger indication at night. Distant signals, usually about 600yd. ahead of the stop signal, the modern name of which is the "home" signal, are so "interlocked" that they can not give a line clear aspect unless the stop signals are previously placed to give the "clear" indication. Before 185o, British rail ways began to adopt the system of working signals and points from a centralized frame in a cabin. In addition to the distant and the stop signals, it has become customary to allow the trains to proceed beyond the home signal to the "starting" signal, also a stop signal (fig. 8), in that all trains must stop at it when it
shows a danger indication. In time of fog, or in snowstorms, it is usual for trains to be stopped at the home signal, operation being assisted by the use of repeater signals, or miniature arms, which repeat the indication given by the semaphore arm, which may be invisible in thick fog. In England, where fog in winter often makes visibility of signals very difficult, fog signalling neces sitates the employment of detonators which are placed on the track near semaphore signals to advise the engine crews of the signal indication. The engine explodes the detonators, or series of detonators, with a loud report. They are put in position by fogmen stationed at the foot of the signal, by means of detonator placing machines. On certain railways, notably in France, a detonator is frequently placed on the rail whenever the stop signal is placed at danger, being automatically withdrawn when the signal gives a "line clear" indication. This provides an engine driver, if he runs past his signal, with the knowledge that he has done so.
Ground signals showing the position of the points were often of the disc type, but the 2oth century has seen a considerable adop tion of the "banner" design, while the miniature semaphore arm is also employed. In Great Britain, because trains always run on the left hand track, the signals are placed, where visibility makes it possible, on the left hand side of the running line, although the railways differ in regard to whether the driver's position is on the left hand side of the engine or not. The signals are operated by means of levers in a signal cabin, which in turn pull a single wire, thus raising a balance weight on the signal, the latter falling through an angle of 6o° in order to give a "line clear" indication. This type is known as the "lower quadrant" design (fig. 9). Manually operated signals in their normal position are kept "on," or at danger, namely at an angle of 9o° with the signal post ; if the wire should break, the balance weight carries the signal auto.