Automatic Train Stop, also called ATS, is a railway safety system designed to prevent certain accidents caused by trains passing stop signals or exceeding speed limits at specific locations.
ATS shares a similar safety goal with Automatic Train Protection, but it usually uses simpler technology and provides less continuous supervision than more advanced ATP systems.
One of the primary purposes of ATS is to help prevent a Signal Passed At Danger, also known as a SPAD. This occurs when a train passes a stop signal without proper authority.
A common ATS design uses a mechanical or inductive trip system. Trackside equipment, such as a trip arm or inductor, is installed near signals or other critical locations.
When a signal displays a restrictive aspect, the trackside ATS device is activated. If a train passes that signal without authorization, onboard equipment interacts with the trackside device and triggers an automatic brake application.
Some ATS systems use inductive loops or radio beacons to transmit speed information at specific locations, such as curves, stations, or areas with permanent speed restrictions.
ATS is often described as an intermittent system because it usually acts only at fixed points, such as signals or speed restriction locations. It does not always provide continuous speed supervision throughout the entire route.
ATS is mainly focused on stopping trains that pass signals at danger and enforcing speed restrictions at critical locations.
Because ATS systems are generally simpler than full ATP systems, they may be less expensive to install and maintain. However, they also provide a more limited level of protection.
ATS remains an important backup safety layer for train crews and signal systems. In many places, ATS has served as a foundation for more advanced train protection systems.