Safety and reliability of comma-free digital coding for railway track circuits

A methodology for quantifying the safety and reliability performance of comma-free codes for railway track circuit data transmission under high levels of Gaussian noise is presented. Two decoding algorithms are described, based on the correct reception of one and two consecutive codewords, respectively. It is shown that use of the second algorithm in conjunction with a subdictionary with a minimum word distance constraint gives comparable safety and reliability performances to those obtained using longer-length synchronisable error-detecting codes. The paper also contains an outline of the properties of traction noise, knowledge of which is necessary for the design of digital railway data transmission systems.