Software requirements for railway signalling systems

A railway signalling system transmits command information to trains. This command information must be interlocked with information relating to train position, state of points and other items of railway equipment to ensure the safety of train movements; processor-based systems are used for these functions. The software for such systems, for example the BR Solid State Interlocking (SSI) and Radio Electronic Token Block (RETB) systems may be considered in terms of a multi-level structure comprising 4 levels. The lowest level, safety management, manages the safety of the system hardware, including self tests and comparisons between parallel redundant processors. The next level is concerned with communications and interfacing. The functional program level implements the rules of the signalling or train control system. It is common to the whole railway network and is configured to the requirements of a specific location by the geographical data level