Automatic Train Stop (ATS) is a railway safety system designed to automatically stop a train under specific unsafe conditions. It acts as a backup safety layer for the train operator.
ATS is primarily designed to prevent trains from passing stop signals without authority and to enforce speed restrictions at specific monitored locations.
One major unsafe condition addressed by ATS is a Signal Passed At Danger, also called a SPAD. This occurs when a train passes a stop signal without proper authorization.
ATS may also enforce speed limits at critical locations such as curves, station approaches, work zones, or other areas where overspeed could create a hazardous condition.
ATS usually operates intermittently. This means it acts at specific points along the railway, such as signal locations or designated speed restriction points, rather than continuously supervising the train along the entire route.
ATS systems use both trackside and onboard equipment. Trackside equipment may include trip arms, inductors, beacons, or other devices. Onboard equipment may include trip cocks, receivers, control units, and brake interfaces.
When a train passes a restrictive signal without authority, the onboard ATS equipment interacts with the trackside equipment. This interaction triggers an automatic brake application.
The automatic brake application is intended to bring the train to a stop before an unsafe movement results in a collision, derailment, or other serious incident.
Compared with continuous Automatic Train Protection systems, ATS is usually simpler and less expensive. However, it provides a more limited level of supervision because it normally acts only at defined locations.
Even though ATS is simpler than some advanced train control systems, it remains an important safety technology for preventing critical operating errors.