AWS

Automatic Warning System. Device used in locomotives of the British Railways network in the early 1950s. Worked by permanent and electromagnets, it sounds a bell in the cab if the signal is green (or semaphore distant signal clear) and a horn if the signal is double yellow, single yellow, or red (or semaphore distant signal at caution). When the horn sounds, the driver has about 3 seconds to press the cancelling button or the brakes will apply. Newer versions of the system are now in operation (ATP and TPWS) but the principle is much the same and helps to prevent a SPAD. (Signal passed at danger).