Automatic Warning System (AWS)

British Rail inherited a number of pre-nationalisation systems in 1948 and developed the Automatic Warning System (AWS), introduced in 1956.

AWS gives train drivers visual and audible warning of the signal aspect displayed by the signal being approached. As the train passes over a magnetic inductor approximately 180 metres from the signal, a visual indicator in the cab turns black.

If the lineside signal is displaying a clear aspect, the indicator stays black and a bell sounds. If it is displaying a cautionary or danger signal aspect, the indicator displays alternate black and yellow sectors and a continuous horn sounds. The driver must cancel the horn within 2 seconds or a full emergency brake application results. The indicator remains black and yellow as a reminder of the last received warning.

AWS provides additional warning of signal aspect, but has limited train protection capability. If the driver cancels the warning, but fails to stop the train, the system will not intervene to prevent the signal being passed. AWS does not distinguish between the various restrictive aspects.

AWS is widespread on the national network, and used on some other railways, but it is not universal. It has been very successful in reducing the risk of signals passed at danger (SPADs) and has almost certainly prevented many accidents.

AWS is also used to warn the driver of some permanent speed restrictions, and all temporary and emergency speed restrictions.

Last updated: 18 February 2008

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