Automatic train stop
An automatic train stop or ATS is a system on a train that will automatically stop a train if certain situations happened (unresponsive train operator, earthquake, disconnected rail, train running over a stop signal, etc.) to prevent accidents from happening. 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 on board 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. 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 70mph.
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 on board 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 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.
Usage around the world
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.
ATS Installations in the United States
|Train stop||NYC Subway||A Division (IRT)||1904-present||Trips right|
|B Division (BMT, IND)||1915-present||Trips left|
|SEPTA||Broad Street Subway||1928-present||Trips left|
|Market–Frankford Line||?-present||Trips left, at wayside signals only|
|MBTA||Blue Line||1925-present||Trips both|
|Orange Line||1901-present||Trips right, at wayside signals only|
|Red Line||1912-present||At wayside signals only|
|Chicago Transit Authority||Chicago 'L'||?-present||Trips left|
|Pennsylvania Railroad/LIRR||New York Tunnel Extension||1911-?||Trips right, used Hall trip valves on trains|
|IIATS||BNSF||ATSF Chicago to Los Angeles "Super Chief" Route||1930's-present||Parts of the route have had ATS removed|
|MetroLink and Coaster||Former ATSF San Diego Main Line.||?-present||In service milepost 179 to 249.|
|New York Central||New York to Chicago Water Level Route||1920's-1970||Removed by successor Penn Central|
|Union Pacific||Former Chicago and North Western Railway North Line, Northwest Line||?-Present||Used by Union Pacific for Metra Commuter trains.|
|New Jersey Transit||RiverLINE||2004-present||Installed at interlockings only. Enforces Stop.|
|Westcab||PAAC||Pittsburgh Light Rail 42S Line from downtown to South Hills Village.||1985-present||Some overlap with an Automatic Train Control system installed on the Route 47 Line.|
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. Below is a list of ATS systems that are specific to Japan only:
- ATS-S(x) (ATS using S-type transponder; always supplemented with: ATS-SF, ATS-SK, ATS-SM, ATS-SN, ATS-SS, ATS-ST or ATS-SW, the last two letters corresponding to the type of transponder used with the S-type transponder)
- ATS-P (ATS using pattern renewal transponder; variants are ATS-PF, ATS-PN, ATS-PT and ATS-Ps) 
- D-ATS-P (Digital ATS-P)
- H-ATS (used on the EF66 locomotive since 1986)
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.
- C-ATS/i-ATS (Used by Keikyu Corporation, Keisei Electric Railway, Shin-Keisei Electric Railway and on Toei Asakusa Line)
- Meitetsu ATS (Used by Meitetsu)
- OM-ATS (Used by Odakyu Electric Railway)
- T-ATS (Formerly used on Toei Mita Line, similar to Tōbu ATS)
- Tobu ATS (Used by Tobu Railway)
- Hankyu Railway, Hanshin Electric Railway, Keio Corporation, Kintetsu, Sagami Railway, Sanyo Electric Railway, Seibu Railway and Tokyu Corporation all use their own proprietary ATS systems.
In Wellington only a few signals at a converging junction are fitted with mechanical ATS. All electric trains are fitted.
- Richey, Albert S. (1915). "Automatic Train Stop". Electric Railway Handbook. New York: McGraw-Hill. pp. 813–815.
- Union Switch and Signal Co. (1911). Automatic Block Signalling for Interurban Electric Railways. Swissvale, PA. p. 33. Bulletin No. 57.
- Comprehensive site about the ATS-S(x) system used in Japan
- Comprehensive site about the ATS-Ps system used in Japan