Automatic warning system (railways)

The Automatic Warning System (AWS) is a form of limited cab signalling introduced in 1956 in the United Kingdom.

The Rail Safety and Standards Board defines it as:

The original concept of AWS was to provide the driver with an audible and visual indication of whether the distant signal was clear or at caution. Should the driver fail to respond to a warning indication, an emergency brake application will be initiated.

Since the introduction of multi-aspect signalling, the majority of signals are fitted with AWS.

It was based on a 1930 system developed by Alfred Ernest Hudd and marketed as the "Strowger-Hudd" system. An earlier contact system, installed on the Great Western Railway since 1906 and known as automatic train control (ATC), was gradually supplanted by AWS within the Western Region of British Railways.

AWS is part of the signalling system and warns the driver whether the next signal is clear or not. These warnings are usually given 200 yards (180 metres) before the signal, though the distance can vary according to linespeed. Information about the signal aspect is conveyed by electromagnetic induction to the moving train through equipment fixed in the middle of the track, known as an AWS magnet. The system works by the train detecting sequences and polarities of magnetic fields passing between the track equipment and the train equipment via a receiver under the train.

The equipment on a train consists of;

As the train passes over an AWS magnet, the 'sunflower' indicator in the driver's cab will change to all black. If the signal being approached is displaying a 'clear' for a semaphore or green for a multiple aspect colour light signal, the AWS will sound a bell (modern locomotives/multiple units use an electronic sounder that gives a distinctive 'ping') and leave the visual indicator black. This lets the driver know that the next signal is showing 'clear' and that the AWS system is working.

If the signal being approached is displaying a restrictive aspect (red, yellow or double yellow in colour-light installations or a distant semaphore at caution (horizontal)), the AWS will sound a horn. The driver then has approximately 2 seconds to press the AWS/TPWS acknowledgement button. The horn then stops and the visual indicator changes to a pattern of black and yellow spokes, which persists until the next AWS magnet and reminds the driver that they have cancelled the AWS. As a fail-safe mechanism, the button must be released after it has been pressed. If a driver should collapse onto the button or keep it held down, the AWS will not be cancelled.

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Wikipedia