Synchronizable error-control coding for railway track circuit data transmission

Digital transmission of cab signalling and train control data to moving traing using railway track circuits may be achieved with FSK modulation of an audio frequency carrier signal. By transmitting coded data, information concerning train location and system status may be carried in addition to safety commands such as target speeds. The distributed nature of railway control systems using track circuits for train detection and track-train data transmission requires the use of coding schemes that tolerate sudden discontinuities at track circuit boundaries in addition to being able to operate in a high noise environment. With audio frequency jointless track circuits, severe constraints are imposed on the transmission frequencies by the characteristics of the electrical separation joint, resulting in low data rates. The paper describes several coding schemes that may be used to overcome the resulting synchronization and error-control problems.