ATP is a system that prevents a train from exceeding its authority e.g. passing a signal at stop (SPAD) or exceeding a speed limit.

Systems typically advise the driver (visually and audibly) of the impending or actual breach. Should the driver fail to bring the train under control, then the train's brakes are automatically applied (penalty brake application), bringing the train to a stand.

There are two types of ATP:

Continuous, where the train borne equipment is constantly updated from a conductor (cable or rail) laid along the block, relaying vital information to the train.

Intermittent, where the train receives vital information via transponders (balises) placed at strategic locations and GSM-R radios.