Automatic Warning System
If the signal being approached is displaying a 'clear' aspect, then AWS will sound a bell tone (modern trains have an electronic sounder that makes a distinctive 'ping') and leave the 'sunflower' black.
If the next signal is displaying a restrictive aspect (e.g. caution or stop) the AWS audible indicator will sound a continuous alarm.
This yellow spoke pattern persists until the train reaches the next AWS magnet and serves as a reminder to the driver of the restrictive signal aspect they passed.
As a fail-safe mechanism, if the driver fails to press the AWS acknowledgement button for a warning indication in sufficient time, the emergency brakes will automatically apply, bringing the train to a stop.
After stopping, the driver can now press the AWS acknowledgement button, and the brakes will automatically release after a safety time out period has elapsed.
AWS works in the same way as for signals, except that a fixed magnet is located at the service braking distance before the speed reduction and no electromagnet is provided (or needed).
Ten years later, Colonel William Yolland of the Railway Inspectorate was calling for a system that not only alerted the driver but also automatically applied the brakes when signals were passed at danger but no satisfactory method of bringing this about was found.
In Germany, the Kofler system used arms projecting from signal posts to engage with a pair of levers, one representing caution and the other stop, mounted on the locomotive cab roof.
To address the problem of operation at speed, the sprung mounting for the levers was connected directly to the locomotive's axle box to ensure correct alignment.
[4] When Berlin's S-Bahn was electrified in 1929, a development of this system, with the contact levers moved from the roofs to the sides of the trains, was installed at the same time.
[citation needed] The first useful device was invented by Vincent Raven of the North Eastern Railway in 1895, patent number 23384.
In 1907 Frank Wyatt Prentice patented a radio signalling system using a continuous cable laid between the rails energized by a spark generator to relay "Hertzian Waves" to the locomotive.
When the electrical waves were active they caused metal filings in a coherer on the locomotive to clump together and allow a current from a battery to pass.
[6] The first system to be put into wide use was developed in 1905 by the Great Western Railway (GWR) and protected by UK patents 12661 and 25955.
Its benefits over previous systems were that it could be used at high speed and that it sounded a confirmation in the cab when a signal was passed at clear.
In the final version of the GWR system, the locomotives were fitted with a solenoid-operated valve into the vacuum train pipe, maintained in the closed position by a battery.
When the signal was at 'caution' or 'danger', the ramp battery was disconnected and so could not replace the locomotive's battery current: the brake valve solenoid would then be released causing air to be admitted to the vacuum train pipe via a siren which provided an audible warning as well as slowly applying the train brakes.
[7] It was possible for specially equipped GWR locomotives to operate over shared lines electrified on the third-rail principle (Smithfield Market, Paddington Suburban and Addison Road).
It was found, however, that the heavy traction current could interfere with the reliable operation of the on-board equipment when traversing these routes and it was for this reason that, in 1949, the otherwise "well proven" GWR system was not selected as the national standard (see below).
[7][8] Notwithstanding the heavy commitment of maintaining the lineside and locomotive batteries, the GWR installed the equipment on all its main lines.
A non-contact method based on magnetic induction was preferred, to eliminate the problems caused by snowfall and day-to-day wear of the contacts which had been discovered in existing systems.
It was successful and British Railways developed the mechanism further by providing a visual indication in the cab of the aspect of the last signal passed.
In a Solid State Interlocking the signal module has a "Green-Proved" output from its driver electronics that is used to energise the electromagnet.
The system is fail-safe since, in the event of a loss of power, only the electro-magnet is affected and therefore all trains passing will receive a warning.
All signal aspects, except green, cause the horn to sound and the indicator disc to change to yellow on black.
To verify that the on-train equipment is functioning correctly motive power depot exit lines are fitted with a 'Shed Test Inductor' that produces a warning indication for vehicles entering service.
Due to the low speed used on such lines the size of the track equipment is reduced from that found on the operational network.
AWS is provided at most main aspect signals on running lines, though there are some exceptions:[1] Because the permanent magnet is located in the centre of the track, it operates in both directions.
