Editing Cab signalling (section)

== Information transmission ==
Cab signals require a means of transmitting information from wayside to train. There are a few main methods to accomplish this information transfer.

=== Electric or magnetic ===
This is popular for early intermittent systems that used the presence of a magnetic field or electric current to designate a hazardous condition.<ref>Railway Signalling – A guide to modern signalling technology, Institution of Railway Signal Engineers. Published 1980.</ref> The British Rail Automatic Warning System (AWS) is an example of a two-indication cab signal system transmitting information using a magnetic field.

=== Inductive ===
[[Inductive sensor|Inductive systems]] are non-contact systems that rely on more than the simple presence or absence of a magnetic field to transmit a message. Inductive systems typically require a beacon or an [[induction loop]] to be installed at every signal and other intermediate locations. The inductive coil uses a changing magnetic field to transmit messages to the train. Typically, the frequency of pulses in the inductive coil are assigned different meanings.  Continuous inductive systems can be made by using the running rails as one long tuned inductive loop.

Examples of intermittent inductive systems include the German [[Indusi]] system.  Continuous inductive systems include the two-aspect [[General Railway Signal]] Company "Automatic Train Control" installed on the [[Chicago and North Western Railroad]] among others.

=== 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.

=== Transponder ===
Transponder based systems make use of fixed antenna loops or beacons (called [[balise]]s) that transmit datagrams or other information to a train as it passes overhead. While similar to intermittent inductive systems, transponder based cab signalling transmit more information and can also receive information from the train to aid traffic management. The low cost of loops and beacons allows for a larger number of information points that may have been possible with older systems as well as finer grained signalling information. The British [[Automatic Train Protection]] was one example of this technology along with the more recent Dutch ATB-NG.

=== Wireless ===
Wireless cab signalling systems dispense with all track-based communications infrastructure and instead rely on fixed wireless transmitters to send trains signalling information. This method is most closely associated with [[communications-based train control]]. [[European Train Control System|ETCS]] levels 2 and 3 make use of this system, as do a number of other cab signalling systems under development.