706 TRANSACTIONS OF SECTION G. 



Central at Newcastle with 494 levers, and the Glasgow Central with 374 levers, 

 and the whole of the Metropolitan District system of underground railways. In 

 this system an air-cylinder is connected to each set of points and to each signal- 

 arm. Air compressed to 65 lb. per square inch is supplied to the cylinders from a 

 main running alongside the railway kept charged by small air-compressors placed 

 at convenient intervals. Each air-cylinder is provided with a small three-way 

 air-valve operated by an electro-magnet. The movement of each air-valve is 

 controlled electrically from the cabin through the electro-magnet associated with 

 it. The system grouped round any one signal-cabin may be regarded as an engine 

 fitted with a large number of cylinders, each working intermittently by com- 

 pressed air, and where in each the valve-rod has been changed to an electric cable, 

 all the cables being led to a signal-cabin where the operation of the valves is done 

 by means of an apparatus which is as easily played upon as a piano, with this 

 difference, however, that the notes are mechanically interlocked so that a signal- 

 man cannot play any tune he pleases, but only a tune which permits of safe traffic 

 movement. Moreover, the instrument is so arranged that the movement of the 

 small lever determining the movement of a signal-arm cannot be completed unless 

 the signal-arm actually responds to the intention of the signalman, thus detecting 

 any fault in the connections between the box and the arm. 



The obvious advantage of power signalling is the large reduction of physical 

 labour required from the signalman. His energy can be utilised in thinking 

 about the traffic movements rather than in hauling all day at signal levers. One 

 man at a power frame can do the work of three at the ordinary frame. The 

 claims made for power signalling, in addition to the obvious advantage of the 

 reduction of labour, are briefly that the volume of traffic which can be dealt with is 

 largely increased, that the area of ground required for the installation is con- 

 siderably less than with the ordinary system, with its rodding, bell-crank levers, 

 chains, and pulleys, and that where the conditions are such that power signalling 

 is justified the maintenance cost is less than with a corresponding system of normal 

 equipment. 



Automatic Signalling. 



Several of the power-signalling installations are automatic in the sense that 

 between signal-cabins on stretches of line where there are no junctions or crossover 

 roads requiring the movement of points, the movement of the signal-arm protecting 

 a section is determined by the passage of the train itself. The most important 

 equipment of this kind is that installed on the group of railways forming the 

 ' Underground ' system. This includes the District Railway with all its branches. 

 On this line the particular system installed is the electro-pneumatic, modified to be 

 automatic except at junctions. Signal-cabins are placed only at junctions and at 

 places where points require to be operated. The stretch of line to be automatically 

 signalled is divided into sections, and the entrance to each section is guarded by a 

 signal-post. Calling two successive sections A and B, the train as it passes from 

 Section A to Section B must automatically put the signal at the entrance to B 

 to danger, and at the same time must pull off the signal at the entrance to A. 

 These operations require the normal position of the signal-arm to be ' off ' instead of 

 at danger, as in the usual practice. The position of the arm in this system conveys 

 a direct message to the driver. If ' on ' he knows that there is a train in the 

 section ; if ' off ' he knows that the section is clear. Each signal-arm is operated by 

 an air motor as briefly described above, but the cables from the valves are now led 

 to relays at the beginning and end of the section which the signal protects. 

 The contrivance by means of which the train acts as its own signalman is briefly 

 as follows. One rail of the running track is bonded, and is connected to the 

 positive pole of a battery or generator. The opposite rail is divided into sections, 

 each about 300 yards long, bonded, but insulated at each end from the rails of the 

 adjacent sections and each section is connected to a common negative main 

 through a resistance. A relay is placed at the beginning and at the end of each 

 section, and is connected across from the positive to the negative rail. Current 

 flows and energises the relay, in which condition the relay completes a circuit to 

 the electro-magnet operating the admission-valve of the air-cylinder on the signal- 

 post, air is admitted, and the signal-arm is held off. This is the normal condition 

 at each end of the circuit. When a train enters a section it short-circuits the 



