British Rail Automatic Warning System (AWS) is an example of a two-indication cab signal system transmitting information using a magnetic field. Inductive 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 Pennsylvania Railroad standard system, a variation of which was used on the London Underground Victoria line, Later, audio frequency (AF) track circuit systems eventually came to replace "power" frequency systems in rapid transit applications as higher frequency signals could self-attenuate reducing the need for insulated rail joints. Some of the first users of AF cab signal systems include the Washington Metro and Bay Area Rapid Transit. More recently, digital systems have become preferred, transmitting speed information to trains using datagrams instead of simple codes. The French 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 balises) 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