Automatic train stop

Automatic train stop or ATS is a system on a train that automatically stops a train if certain situations occur (unresponsive train operator, earthquake, disconnected rail, train running over a stop signal, etc.) to prevent accidents. In some scenarios it functions as a type of dead man's switch. Automatic train stop differs from the concept of automatic train control in that ATS usually does not feature an onboard speed control mechanism.

The invention of the fail-safe railway air brake provided an external means for stopping a train via a physical object opening a valve on the brake line to the atmosphere. Eventually known as train stops or trip stops, the first mechanical ATS system was installed in France in 1878 with some railroads in Russia following suit using a similar system in 1880.^{[citation needed]} In 1901 Union Switch and Signal Company developed the first North American automatic train stop system for the Boston Elevated Railway. This system was soon adopted by the New York City Subway and other rapid transit systems in the United States.

Mechanical ATS was more popular on rapid transit systems and dedicated commuter rail than freight or long-distance passenger lines due to a combination of the increased complexity found in mainline railroad operations, the risk of inadvertent activation by debris or other wayside appliances, and the danger of emergency brake applications at high speeds. Moreover, the forces involved in a physical tripping action can begin to damage both the wayside and vehicle borne equipment at speeds over 70 miles per hour (110 km/h).

In 1910 the Pennsylvania and Long Island Rail Roads installed a mechanical ATS system covering various lines to New York Penn Station using the patented Hall trip valve which was designed to prevent inadvertent activations from debris, however the system was only installed on locomotives and multiple units traveling to Penn Station and did not see further adoption.

While similar in operation mechanical systems around the world are generally incompatible due to the wide variety of vehicle dimensions and track gauge which will result in the mechanical stopping devices not engaging the onboard valve.

Electronic systems make use of electric currents or electromagnetic fields to trigger some action in the locomotive cab. While mechanical systems were generally limited to venting the brake pipe and triggering an emergency stop, electronic systems can trigger other actions such as an acknowledgment from the driver, cutting power or a less severe application of the brakes. Without physical contact electronic systems could be used with higher speeds, limited only by the equipment's ability to sense the signal from stop devices.

The first such electronic system was Crocodile (train protection system) installed on French railways starting in 1872 which used an electrified contact rail to trigger an acknowledgment from the driver. If no such acknowledgment was made in 5 seconds the train would be stopped. In the UK the Great Western Railway implemented a similar system in 1906 dubbed Automatic Train Control that served as the template for the magnetic based Automatic Warning System, which ultimately replaced it starting in the 1950s.

In the United States, the General Railway Signal corporation introduced its Intermittent Inductive Automatic Train Stop system in the 1920s which made use of inductive loops in a "shoe" mounted outside of the running rails. This system was also of the acknowledgment type and was adopted by several railroads, continuing to see service as of 2013.