This standard defines the protocol for Inter-car and Intra-car serial data communications between subsystems aboard passenger trains. It sets forth the minimum acceptable parameters for a network that can simultaneously handle monitoring and controls traffic from multiple systems. While the network itself is not vital, it is intended to be capable of carrying vital messages. The standard will be structured with respect to the OSI seven layer model. To permit systems provided by a variety of suppliers performing different onboard functions to share a common communications facility. Car wiring and weight will be reduced while systems integration, reliabiity and maintability will be improved. TCRP project G-4 has estimated an expected value of $56.2 million/year savings. New IEEE Standard - Superseded. Communications protocols to be used for intercar and intracar serial data communica-tions between subsystems aboard passenger trains are defined by this standard. Minimum accept-able parameters for a network that can simultaneously handle monitoring and control traffic from multiple systems are set forth. While the network is not vital, it is intended to be capable of carrying vital messages.