It was realised prior to 1914, that that some sort of automatic and enforceable signal warning would significantly improve rail safety. A number of experimental systems were trialed and eventually a system with no physical contact between the train and track was chosen. This became known as the Automatic Warning system, or AWS as it is often referred to today.
The AWS system has two elements, an on train AWS control unit and an on track magnet, known as the ‘AWS Ramp’.
An ‘AWS Ramp’ consists of a pair of magnets, the first is permanent and the second an electro-magnet that’s wired to a signal. The ‘AWS Ramp’ is placed between the rails and a receiver fitted to the train detects the active magnetic fields.
The AWS Ramp, is set at a distance of 185 metres (200 yards), on the approach side to a signal. When the AWS receiver detects a magnetic field the driver is ‘Automatically Warned’ of the presence and status of the approaching signal by the ‘System’.
These warnings are audible and consist of a single ‘ping’ for a signal that is at a ‘green aspect’ and a continuous ‘horn’ for a signal that is set to an ‘amber or red aspect’. The train driver must acknowledge the continuous ‘horn’ within 2.7 seconds otherwise the vehicle brakes are automatically applied by the onboard AWS control unit. The brakes are applied to retard the progress of the train and bring it to a halt, so preventing a possible collision with any opposing rail traffic.
A visual indicator (drivers reminder), known as the ‘AWS Sun-flower’ also forms part of the overall, Automatic Warning System. This indicator stores the status of the signal that the train is approaching. It is set to ‘all black’ for a ‘green aspect’ or ‘black and yellow’ for an amber or red aspect.
The AWS receiver is mounted on the leading bogie of the train, and is connected via the junction box to the TPWS control unit mounted within the cab.
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