UK Railway Signalling - Warning Systems

Warning Systems

Because of the propensity for heavy fog in some parts of the British Isles, fog signal rules were established on the UK railway system to keep train traffic moving without incurring the severe delays that would be necessary if drivers had to stop or travel slowly up to each signal and read its indication. During heavy fog, fogsignalmen would be stationed at distant signals with a lantern and detonators — small explosive charges that could be strapped to the rail to be exploded by the wheels of a train. The fogsignalman's duty was to repeat the indication of the signal using his lantern; the semaphore arm was usually obscured by fog and hence invisible to the driver of a moving train. If the distant signal were displaying 'caution' (warning that a signal ahead was at 'danger'), the detonators remained on the rail and the fogsignalman would show a yellow lamp to show 'caution'; if the distant signal were clear, the detonators would be removed from the rails and a green lamp would be displayed.

Britain's Great Western Railway introduced the Automatic Train Control (ATC) system in 1906. This system is the forerunner of today's Automatic Warning System (AWS) and consists of an electrical system that sounded a bell in the cab as the train approached a signal at clear. Power was fed through a metal ramp to a pickup on the underside of the locomotive to power the bell. An absence of the electrical voltage on the ramp caused a warning horn to sound in the locomotive's cab. The driver then had a set time to acknowledge the warning and start braking his train accordingly. If the driver did not acknowledge the warning, the brakes would be applied automatically. Where this was implemented, it did away with the need for fog signalling, since the driver could tell the state of the distant signal regardless of his ability to see it.

The current system of AWS in use on Britain's railways is similar in principle to the Great Western's ATC but does not rely on physical contact between the track equipment and the train; instead an inductive system is used.

On passenger lines, AWS is now often supplemented by the Train Protection & Warning System (TPWS). TPWS will automatically apply the train's brakes in the event of a fitted signal being passed at danger without authority.