Editing Cab signalling (section)

=== Coded track circuits ===
A coded track circuit based system is essentially an inductive system that uses the running rails as information transmitter. The coded track circuits serve a dual purpose: to perform the train detection and rail continuity detection functions of a standard [[track circuit]], and to continuously transmit signal indications to the train. The coded track circuit systems eliminate the need for specialized beacons.

Examples of coded track circuit systems include the [[Pulse code cab signaling|Pennsylvania Railroad standard system]], a variation of which was used on the London Underground [[Victoria line]],<ref>{{cite web| work=Tubeprune| title=Automatic Train Operation on the Victoria Line| url=http://www.trainweb.org/tubeprune/Victoria%20Line%20ATO.htm| date=2003-03-15| access-date=2008-03-13}}</ref> Later, [[audio frequency]] (AF) track circuit systems eventually came to replace "power" frequency systems in rapid transit applications as higher frequency signals could self-[[attenuation|attenuate]] reducing the need for insulated rail joints. Some of the first users of AF cab signal systems include the [[Washington Metro signaling and operation|Washington Metro]] and [[Bay Area Rapid Transit]]. More recently, digital systems have become preferred, transmitting speed information to trains using [[datagram]]s instead of simple codes. The French [[Transmission Voie-Machine|TVM]] makes use of the running rails to transmit the digital signalling information, while the German [[LZB]] system makes use of auxiliary wires strung down the centre of the track to continually transmit the signalling information.