Amazingly, all of these groups have independently specified the same standard communications protocol. But before going further, it's appropriate to review the history of trainline communications to better understand where these new standards fit in and why they are so important. Until recently it was standard rail transit practice to dedicate one or more electric train line wires to control or monitor each train function. For example, in a transit vehicle consist, one pair of trainlines might control all left side doors and another pair the right side. Door status also requires a pair of trainlines to report all left side doors closed and another pair is needed for the right side status.
Doors are just one of many trainlined subsystems.
More wires also mean more electric coupler pins, and that means higher failure rates. But wait, it gets worse. New Technology Headaches San Francisco's new streetcars are typical of new high technology cars that do not properly benefit from serial train lines. Then, similar information must be entered into the Motorola digital radios. After that, route data must be entered into Vetag transponder boxes to control surface track switches and similar information must be entered for subway operation using the new Alcatel Communications Based Train Control System.