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An 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 does not feature an onboard speed control mechanism.
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Mechanical systems
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. 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 value which was designed to prevent inadvertent activations from debris, however the system was only installed those 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
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 be the equipment's ability to sense the signal from stop devices.
The first such electronic system was Le Crocodile 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.
In 1954, Japan introduced ATS-B, the first known variant of ATS. In 1967, ATS-S (and its various supplements) was invented, the first non-contact-based ATS to be used; in 1974, ATS-P was used for the first time, and in 1986, H-ATS was invented.
United States
The majority of systems meeting the definition of Automatic Train Stop in the United States are mechanical trip stop systems associated with rapid transit lines built in the first half of the 20th century. Since 1951 ATS has been required by the Interstate Commerce Commission (later the Federal Railroad Administration) as a minimum safety requirement to allow passenger trains to exceed a speed limit of 79 mph (127 km/h). The regulatory requirement refers to a system that triggers an alert in the cab of the locomotive whenever the train passes a restrictive wayside signal and that then requires the locomotive engineer to respond to the alert within a set period of time before the brakes are automatically applied.
The most popular implementation of ATS for the mainline railroad industry was made by the General Railway Signal company starting in the 1920s and consisted of inductive coils mounted just outside the right hand rail in relation to the direction of travel. Often referred to as just ATS in railroad operating books, the full name of the system is Intermittent Inductive Automatic Train Stop to differentiate it from mechanical systems being offered at the time. The popularity of ATS as a train protection mechanism fell after the introduction of track coded cab signals in the 1930s.
Japan
Many trains in Japan are equipped with this system. The ATS systems in Japan are slightly similar to those used in the United States, but are mostly transponder-based. The first ATS systems in Japan were introduced in the early 20th century, but did not become commonplace until the late 1960s as a result of the Mikawashima train crash which occurred in 1962. Below is a list of ATS systems that are specific to Japan only:
JR Group
Private railways/Subway lines
In addition, various private-sector railways and subway lines have adopted their own versions of the ATS system since 1967. Like the ATS systems used by the railways in the JR Group, they are transponder-based as well, but are generally incompatible with the ATS systems used by JR.
New Zealand
In Wellington only a few signals at a converging junction are fitted with mechanical ATS. All electric trains are fitted.
South Korea
Some Korail and subway lines are equipped with this system.
Argentina
Underground lines and have ATS equipped, while , , and have the more advanced Communications-based train control.
Taiwan
Many Taiwan Railways Administration trains are equipped with the Japanese ATS-SN and ATS-P systems, which serve as fallback for an ATP system introduced in 2006, of which the latter system replaced the older AWS system originally introduced in 1978 on some express trains.
United Kingdom
Some Manchester Metrolink services are ATS equipped.
London Underground lines are universally fitted with ATS equipment. This comprises a trip arm outside the left-hand running rail, and an air valve known as a tripcock on the leading bogie of the train. When the applicable signal shows 'danger', the trip arm is held up by a spring. If a train attempts to pass the signal, the trip arm makes contact with the tripcock. This opens the tripcock, which is connected to the train pipe of the air brakes, and causes an emergency brake application to be made. When the signal shows 'clear', the stop arm is lowered by compressed air.