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The Automatic Warning System (AWS) 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. It was based on a 1906 system developed by the Great Western Railway. 1948 (MCMXLVIII) was a leap year starting on Thursday (the link is to a full 1948 calendar). ...
1906 (MCMVI) was a common year starting on Monday (see link for calendar). ...
The original Bristol Temple Meads station, first terminus of the GWR, is the building to the left of this picture The Great Western Railway (GWR) was a British railway company, linking South West England, the West Country and South Wales with London. ...
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. A signal is a mechanical or electrical device that indicates to train drivers information about the state of the line ahead, and therefore whether he or she must stop or may start, or instructions on what speed the train may go. ...
In physics, magnetism is one of the phenomena by which materials exert an attractive or repulsive force on other materials. ...
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. The Train Protection & Warning System (TPWS) has been deployed across the entire UK railway network. ...
British Rail AWS The British Rail AWS consists of: Logo of British Rail British Railways (BR), later rebranded as British Rail, ran the British railway system from the nationalisation of the Big Four British railway companies in 1948 until its privatisation in stages between 1994 and 1997. ...
- a permanent magnet between the rails
- an electro-magnet between the rails (with opposite pole to the permanent magnet)
- a cab indicator that can show a black disk or a yellow and black "exploding" disk. Sometimes known as the "AWS sunflower"
- a control unit that connects the system to the brakes on the train.
When the train approaches 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. Klaxon is a trademark for an electromechanical horn or alerting device. ...
Track brakes are a form of braking unique to railbourne vehicles. ...
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.
The system is failsafe, since in the event of a loss of power, only the electro-magnet is effected, and therefore trains passing will recieve a warning.
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 stops. All signal aspects, except green, cause the klaxon to sound and the indicator disc to change to yellow on black. A Train Stop is a mechanical or electrical device that stops a train if it attempts to pass a red signal, or if it attempts to pass at an excessive speed. ...
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. This article is about the unit of information. ...
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, this has led to a number of fatal accidents. Also, there is no compulsory stop when a red signal is passed. The newer TPWS overcomes some of these problems. The Train Protection & Warning System (TPWS) has been deployed across the entire UK railway network. ...
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. The original Bristol Temple Meads station, first terminus of the GWR, is the building to the left of this picture The Great Western Railway (GWR) was a British railway company, linking South West England, the West Country and South Wales with London. ...
1906 (MCMVI) was a common year starting on Monday (see link for calendar). ...
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.
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