Editing Automatic Warning System (section)

=== 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&nbsp;V or more; the [[Electrical resistance and conductance|resistance]] if the power came directly from the controlling signal box was thought too great (the locomotive equipment required 500&nbsp;[[milliamp|mA]]). Instead, a 3&nbsp;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.<ref name=awjd>{{cite journal |last1=Dymond |first1=A. W. J. |title=The Automatic Train Control System of the Great Western Railway |journal=Transactions |date=10 March 1936 |issue=206 |pages=102;115|publisher=G. W. R. Swindon Engineering Society}}</ref>

It was possible for specially equipped GWR locomotives to operate over shared lines [[Railway electrification in Great Britain|electrified]] on the third-rail principle ([[Smithfield Market]], [[Paddington tube station (Circle and Hammersmith & City lines)#Metropolitan Railway|Paddington Suburban]] and [[Kensington (Olympia) railway station|Addison Road]]). At the entrance to the electrified sections a particular, high-profile contact ramp ({{cvt|4+1/2|in|disp=sqbr}} instead of the usual {{cvt|2+1/2|in|disp=sqbr}}) 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).<ref name=awjd/><ref name=grsw/>

Notwithstanding the heavy commitment of maintaining the lineside and locomotive batteries, the GWR installed the equipment on all its main lines. For many years, [[Western Region of British Railways|Western Region]] (successors to the GWR) locomotives were dual fitted with both GWR ATC and BR AWS system.