automatic warning system

The Automatic Warning System refers to the specific form of limited cab signalling introduced in 1948 in the United Kingdom to help train drivers observe and obey warning signals, yellow or green.

BR AWS

The BR AWS consists of:

a permanent magnet between the rails
an electro-magnet between the rails
a cab indicator that can show a black disk or a yellow and black "exploding" disk.
a control unit that connects the system to the brakes on the train.

When the train passes a distant signal at yellow, the electro-magnet is de-energised, the indicator disk changes to yellow on black, and a hooter sounds. If the driver fails to acknowledge the warning, the brakes are applied. When the train passes a distant signal at green, the electro-magnet is energised, the indicator disk changes to all black, a bell chimes. No acknowledgement is required from the driver. With mechanical signalling, the AWS system was only installed at distant signals, but with multi-aspect signalling, the AWS is fitted at all main line signals.

Disadvantages

The AWS has a very low bandwidth, only 2 bits of information. Even though a multi-aspect signal can display several bytes of information, the AWS can only distinguish between 2. The AWS is an advisory system, and can be easily overridden by habituated reactions of the driver, especially when the driver proceeds on multiple yellow signals. Also, there is no compulsary stop when a red signal is passed. The newer TPWS overcomes some of these problems.

GWR AWS

The GWR AWS developed circa 1906 is very similar to the BR one, the main difference being that it used a contact ramp rather than non-contact magnets. This ramp was located on the approach to the distant signal, and pushed up slightly higher than the level of the rail on a plunger mounted on the engine. If the ramp was engergized, that gave the green signal. If the ramp was de-energized, that gave the yellow signal. For many years Western Region locomotives were dual fitted with both GWR and BR AWS system, and the GWR system was gradually phased out. The GWR system's main disadvantage is the wear and tear on the ramp and the plunger, which is avoided with the non-contact BR version.