An overlay on existing signal and main track authority systems mandated by Congress and implemented in the mid to late 2010’s. It is implemented on routes that carry passenger trains and that carry a significant amount of hazardous material.
PTC is a system that prevents a train from exceeding its authorized speed, collisions with other trains at speed, encountering an open main track switch at speed or exceeding its main track authority. Unlike previous systems that stopped a train after it exceeded its authority, PTC is designed to be proactive and stop the train before it exceeds it authority. It also monitors speed on a dynamic basis.
The system relies on information from the railroads information computer system (train route, physical characteristics of the track such as grade or switch/station locations, train length, train weight), information from the dispatching system (authority limits granted, signal indications, which track the train on, speed restrictions), the status of the track (switches open) and the operating parameters of the train (speed, acceleration, location). The information is gathered through a radio network and relayed to the train. The system on the train determines if the train is operating at or below the proper speed and if the train has enough room to stop before it reaches the end of its authority. If the train is operating properly, the system takes no action. If the train is exceeding authorized speed or is not taking action to stop or reduce speed where required, the system will stop the train.
PTC is not a cruise control, it does not automatically run the train. All it does is stop the train if the train is speeding, approaching a collision or not stopping at the end of authority. It is all about “stop” and has nothing to do with “go”.
There are other systems on engines that automate the operation and can assist or handle the “go” part.