matically to the "on" position. The use of double wires to operate the signals permits the elimination of balance weights, thus mak ing the movement over a long distance easier for the signalman, but double wire signalling, which has been common on railways in Continental Europe and the Dominions, has not met with great favour in the past in Great Britain. The L.M.S.R. and the L.N.E.R. have expressed the intention of adopting "upper quad rant" signals (fig. ro and i 1), in the case of new construction, which to give the "off," or "clear" indication will be raised, instead of falling, through at least 60°. By the adoption of this "upper quadrant" design considerable economies are-possible in connec tion with the rodding and balance weights on the signal post, which may be made of wood, reinforced concrete, metal poles or metal lattice work.
In addition to manual operation, points and signals may also be operated by electricity and compressed air, first used on the British railways by Sykes in 1875, or by a combination of both these systems. Meanwhile mechanical interlocking has given place in many installations to power interlocking of the all-electric or electro-pneumatic type, although in certain cases hydraulic power has been employed.
This followed the inauguration of the electric telegraph line from Paddington to West Drayton in 1839, and was improved and extended to Slough by 1842, while two years later the single line Norfolk Railway was worked by telegraph from a central office. By 1854 the L.N.W.R. had adopted a three indication block signalling instrument which showed "line blocked," "line clear," and "train on line." The block system in essence consists of the refusal to permit a train to enter a suc cessive section of line until the preceding train is clear of that section ; with manual signal operation and control, movements in each section are controlled from one signal box. The block system, in replacing the time interval by a space interval, greatly increased the degree of safety. The simple block system was further im proved and much greater safety obtained by the later adoption of the "lock and block" system, due largely to W. R. Sykes about 1875. The lock and block system prevented the signalman from placing the signals in the clear position for a second train until the preceding one was clear of the section; this was effected by means of locking the mechanical signals with electrical block instruments, which did not permit the movement of the starting signal at A until the lock was released by the signalman at the succeeding signal box B. In certain cases the wheels of the train, acting in connection with an electrical current passing through the rails and known as a "track circuit," releases by its own passage past a certain point the lock in the signal cabin. This "track circuit" is the essence of automatic signalling to be described later.
The lock and block system has contributed largely to the safety of British railways, the "absolute" block, namely rigid adherence to the block system, being standard on all passenger lines; the "permissive" block, which allows succeeding trains in the same section when under full control, is only admissible in stations and on lines used by freight trains. In countries other than Great Britain, including the United States, the "permissive" block system is, however, allowed on lines used by passenger trains. On single
lines safety is obtained by the employment of a staff or "tablet" which is carried on the engine during its passage of the section. The Tyers tablet system of 1878 permitted, by means of the block system, the entry of a train at either end of the section, an impos sibility where a single wooden staff was employed. By 1890 the "electric train staff" had been invented, which was an improve ment on the tablet and permits successive trains to enter from either end of the section, but only one tablet can be withdrawn at a time, and that not until the preceding tablet for the section has been replaced at one end or the other.
was first installed by the Liverpool Overhead (of England) in 1893. At that time the track circuit (see Block System above) was hardly known. It consists in its simplest form of a section of the running rails, insulated from the rails on either side, being fitted with a battery at one end and a relay at the other, so connected that the current may cir culate from the battery along one running rail through the relay and back to the battery along the other running rail. This current energizes the relay coils which pick up the arma ture, thus effecting contacts which remain closed until the two running rails are connected through the wheels and axle of a train. Signals controlled by circuits were first installed on the L.S.W.R. in 1902, and also adopted later on a section of the North Eastern Railway. Automatic signalling led to the use of day "colour light" signals (figs. 12 and 13) em ployed by the Liverpool Overhead in 1921 ; such a system is also used giving more than two as pects, thus the "light" signals be tween Marylebone and Wembley give a "danger," "caution" or "clear" indication ; while the S.R. has gone one step further by pro viding a fourth aspect, namely two yellow lights on certain of its lines with the densest traffic. The semaphore signal can also give three aspects, namely
and vertical, with the post, but in this case it must be of the "upper quadrant" type, such as has been in use at certain London stations and elsewhere since
Light signals are rapidly gain ing in favour owing to their lower cost of installation, better visibil ity and identical aspect by day and night ; also they require less space and can be placed on a level with the driver's eye. Mod ern signalling rests largely upon the use of the track circuit and electrical devices, which permit the use of long distance point and signal operation with adequate electrical "detectors" and "repeaters," while even level crossing gates are sometimes electrically operated and frequently electri cally interlocked with the signals. One large electrically operated signal box may replace as many as five older mechanical manually operated boxes; thus, signalling is rapidly becoming one of the most important departments on a railway, partly owing to the great operating economies which it is able to make, and partly owing to the necessity of making the best use of tracks and roll ing stock. Signals, at first installed to prevent collisions, have also become the means whereby a maximum traffic may be handled at a maximum speed with minimum costs, brought about by the aid they give in keeping the trains moving.