It was a way of showing in the locomotive cab, if signals obscured by fog or falling snow, were indicating “Clear”, “Caution” or “Danger”. The North Eastern Railway had experimented with something similar before WWI, with a test installation between Darlington and Richmond, which they correctly described as “Cab Signaling”. It was an electro mechanical device requiring a plunger on the locomotive to make physical contact with a lengthy ramp carrying an electrical current, laid between the rails. The Great Western developed something similar, and had equipped all their main lines by 1939. The drawback was the mechanical element, which might suffer breakage or poor contact in icy conditions, giving a “fail safe” danger indication causing unnecessary delay, and if the locomotive pick up were damaged, then the engine had to be taken out of service for repairs.
A system using electro-magnetic induction, avoiding physical contact between a much shorter ramp and the loco pick-up (the American “Hudd” System) was trialed by the LMS on the London Tilbury and Southend line in 1947, and it was an adaptation of this which was adopted as the British Railways standard under the provisions of the 1955 Modernisation Plan. AWS has itself now been replaced by a more sophisticated electronic system.
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