Electric Signalling and Operation.

A signal-box equipped with a frame containing 312 levers is a sight which would create rare wonder in the minds of George Stephenson and his fellow pioneers, were they alive to-day. A cabin of this size is now being installed by the Southern line at London Bridge Station, in the metropolis, and this new signal-box will rank as the third largest signal cabin in Britain, the page 20 lead being taken by the St. Enoch Station box in Glasgow with 488 levers, and the Central Station cabin in the same city with 374 levers. The new power-operated box at London Bridge takes the place of nine existing cabins, and enables a saving to be effected of 35 signalmen, four signal lads and five signal lampmen. Some two and a half miles of track are controlled from London Bridge to Bricklayers Arms, containing eleven sets of metals, as well as the intricate station working. Three aspect colour light signals are employed on this section, and all the tracks covered are reproduced on a large diagram in the box, whereon is recorded the progress of trains by the aid of coloured lights.

While the Southern line, in common with the other trunk railways serving London, is faced with very perplexing signalling problems, it is on the Underground railways of the capital that the most wonderful of signalling systems has been devised to ensure the safe working of the exceptionally dense traffic handled over the surface and underground tracks which cross and criss-cross throughout the city and suburbs. On all the lines constituting the London underground system power signalling is installed. All lines are track-circuited throughout, the single rail type being favoured in all instances, except on the Central London route where both running rails are employed for traction current return and double rail track circuits with impedance bonds are favoured. The signals, operated from power frames fitted with miniature levers, are of the semi-automatic pattern, and are replaced to “danger” when a train passes them, irrespective of the position of the lever. After being put to “danger” the signals will not return to the “clear” indication until the lever controlling them has been replaced to the normal position in the power frame, and again reversed. This is effected by a “stick” relay, and the back locking circuits for the electric lock on the signal lever also are taken through this relay.