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AWS links to the Signalling Systems to warn the driver about the aspect of the next signal. These warnings are typically 200 yards before the signal. Information about the signal aspect is conveyed by Magnetism to the moving train through boxes fixed in the middle of the track, known as ''AWS ramps''. When the AWS ramp is reached, the AWS sets the visual indicator to all black. If the signal being approached is set to clear, the AWS will sound a bell and leave the indicator black. This lets the driver know that the next signal is set to clear and that the AWS system is working. If the signal being approached is set to a restrictive aspect (red, yellow or double yellow), AWS will sound a horn continuously until the driver pushes a button to disarm it. If the six second countdown passes and the button is not pressed, a full emergency brake application is made to bring the train to a halt. When the warning is acknowledged, the horn stops and the indicator changes to a pattern of black and yellow spokes. AWS (then called ATC, Automatic Train Control) was first used in the days of steam on the North Eastern Railway and then the Great Western Railway. Its potential to stop accidents was quickly seen and it was eventually adopted nationwide. AWS also works with mechanical signalling methods, where it is only used at distant signals. The latest system which is being used to augment AWS is the Train Protection & Warning System . BRITISH RAIL AWS The British Rail AWS consists of:
When the train passes a distant signal at yellow, the electro-magnet is de-energised and a Klaxon warning is given until the driver acknowledges. If the driver fails to acknowledge the warning within a pre-determined time period, the friction axle brakes or (if fitted) Track Brakes are applied. If the driver does acknowledge, the indicator disk changes to yellow on black, to remind the driver that he has acknowledged a warning and has manual control of the brakes. When the train passes a distant signal at green, the electro-magnet is energised, the indicator disk changes to all black, a bell (electro-mechanical or electronic) is sounded. 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 and sometime used in conjunction with Train Stop s. All signal aspects, except green, cause the claxon to sound and the indicator disc to change to yellow on black. Disadvantages The AWS has a very low bandwidth, only 1 Bit of information. Even though a multi-aspect signal can display several bits of information, the AWS only has two states. 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 compulsory stop when a red signal is passed. The newer TPWS overcomes some of these problems. Great Western Railway ATC The Great Western Railway ATC (Automatic Train Control) developed in 1906 is very similar to the British Railway system, 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 energized, 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 ATC 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. SEE ALSO
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