Automatic Train Control (ATC) is a railway safety and train management system that provides a higher level of protection than Automatic Train Stop (ATS). ATC continuously monitors train operation and helps ensure compliance with signal indications, speed restrictions, and movement authorities.
Unlike ATS, which typically acts only at specific locations, ATC provides continuous or near-continuous supervision of train movement. This allows the system to monitor train performance throughout the route rather than only at selected points.
ATC systems continuously exchange information between trackside equipment and onboard train equipment. This communication provides the train with information about signal conditions, speed limits, movement authority, track gradients, and other operating requirements.
One of the most important functions of ATC is continuous speed supervision. The system constantly compares the train's actual speed against the maximum permitted speed for the current operating conditions.
If the train exceeds the permitted speed profile, the ATC system can provide warnings to the train operator. If corrective action is not taken, the system can automatically apply the brakes.
ATC also enforces signal compliance. If a train approaches a restrictive signal too quickly or attempts to pass a signal without authority, the system can intervene to prevent unsafe operation.
Many ATC systems integrate cab signaling. Instead of relying solely on wayside signals, signal information is displayed directly inside the locomotive cab where it is continuously visible to the train crew.
Advanced ATC systems calculate dynamic braking curves. These calculations consider train speed, train weight, track grade, braking performance, and distance to restrictions to determine when braking must begin.
Various technologies are used for ATC communications, including coded track circuits, inductive loops, transponders, balises, and radio-based communication systems.
By continuously supervising train movement and automatically enforcing operating limits, ATC significantly improves railway safety while allowing railroads to operate trains more efficiently and with greater capacity.