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