Early devices in its engaged (left) and disengaged (right) position Early devices used a mechanical connection between the signal and the locomotive. In 1840, the locomotive engineer
Edward Bury experimented with a system whereby a lever at track level, connected to the signal, sounded the locomotive's whistle and turned a cab-mounted red lamp. 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 1873, United Kingdom Patent No. 3286 was granted to Charles Davidson and Charles Duffy Williams for a system in which, if a signal were passed at danger, a trackside lever operated the locomotive's whistle, applied the brake, shut off steam and alerted the guard. Numerous similar patents followed but they all bore the same disadvantage – that they could not be used at higher speeds for risk of damage to the mechanism – and they came to nothing. 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. 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. The first useful device was invented by
Vincent Raven of the
North Eastern Railway in 1895, patent number 23384. Although this provided audible warning only, it did indicate to the driver when points ahead were set for a diverging route. By 1909, the company had installed it on about 100 miles of track. 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. The signal was turned off if the
block were not "clear"; no current passed through the coherer and a
relay turned a white or green light in the cab to red and applied the brakes. The
London & South Western Railway installed the system on its
Hampton Court branch line in 1911, but shortly after removed it when the line was
electrified.
GWR automatic train control 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. At each distant signal, a long ramp was placed between the rails. This ramp consisted of a straight metal blade set edge-on, almost parallel to the direction of travel (the blade was slightly offset from parallel so in its fixed position it would not wear a groove into the locomotives' contact shoes), mounted on a wooden support. As the locomotive passed over the ramp, a sprung contact shoe beneath the locomotive was lifted and the battery circuit holding closed the brake valve was broken. In the case of a clear signal, current from a lineside battery energising the ramp (but at opposite polarity) passed to the locomotive through the contact and maintained the brake valve in the closed position, with the reversed-polarity current ringing a bell in the cab. To ensure that the mechanism had time to act when the locomotive was travelling at high speed, and the external current therefore supplied only for an instant, a "slow releasing relay" both extended the period of operation and supplemented the power from the external supply with current from the locomotive battery. Each distant signal had its own battery, operating at 12.5 V or more; the
resistance if the power came directly from the controlling signal box was thought too great (the locomotive equipment required 500
mA). Instead, a 3 V circuit from a switch in the signal box operated a
relay in the battery box. 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. The driver was then expected to cancel the warning (restoring the system to its normal state) and apply the brakes under his own control - if he did not the brake valve solenoid would remain open, causing all vacuum to be lost and the brakes to be fully applied after about 15 seconds. The warning was cancelled by the driver depressing a spring-laden toggle lever on the ATC apparatus in the cab; the key and circuitry was arranged so that it was the lever returning to its normal position after being depressed and not the depressing of the lever that reset the system - this was to prevent the system being overridden by drivers jamming the lever in the downward position or the lever accidentally becoming stuck in such a position. In normal use the locomotive battery was subject to constant drain holding closed the valve in the vacuum train pipe so to keep this to a minimum an automatic cut-off switch was incorporated which disconnected the battery when the locomotive was not in use and the vacuum in the train pipe had dropped away. 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). At the entrance to the electrified sections a particular, high-profile contact ramp ( instead of the usual ) raised the locomotive's contact shoe until it engaged with a ratchet on the frame. A corresponding raised ramp at the end of the electrified section released the ratchet. 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). It was tested by the
Southern Railway,
London & North Eastern Railway and the
London, Midland & Scottish Railway but these trials came to nothing. In 1947, Hudd, now working for the LMS, equipped the
London, Tilbury and Southend line, a division of the LMS, with his system. Following the
Nationalisation of Britain's railways and the creation of
British Railways (BR) in 1948, experimental work continued. A requirement was formulated for a standardised AWS solution that would be suitable for deployment across all parts of the British railway network, operate under all weather conditions, and work with all forms of signalling and traction then in use, including on electrified lines. By mid-1959, the AWS system had been partially deployed, having become operational between Kings Cross and York, Euston and Rugby, and Edinburgh and Glasgow. • A
colour light signal displaying a double yellow (steady or flashing), single yellow or red aspect • A
reduction in permissible speed • A
temporary or emergency speed restriction • An
automatic barrier crossing locally monitored (ABCL), an
automatic open crossing locally monitored (AOCL), or an
open crossing (OC). AWS was based on a 1930 system developed by Alfred Ernest Hudd and marketed as the "Strowger-Hudd" system. An earlier contact system, installed on the
Great Western Railway since 1906 and known as
automatic train control (ATC), was gradually supplanted by AWS within the
Western Region of British Railways. == Network Rail ==