An automatic train stop 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.
The first (mechanical) ATS system was installed in France in 1878. In 1880, most of the (later) Soviet Union started using ATS on its railway lines, the second country to be fitted with ATS. In 1921, Japan started using ATS on the Tokaido Main Line, the third country to do so.
Great Britain and America started using ATS in the 1920s, Germany adopted it in the 1930s and China first used it in the 1950s. By 1980, almost every railway used some form of ATS.
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.
This is one of the systems prescribed since 1951 by the federal government to allow passenger trains to exceed a speed limit of 79 mph (127 km/h). The term applies to an intermittent system that triggers an alerter in the cab of the locomotive that the engineer must respond to within a set period of time before the brakes are automatically applied. The system has no ability to enforce speeds or signal indications, only the the attention of the engineer.
The most popular implementation of ATS was made by the General Railway Signal company and consisted of inductive coils mounted just outside the right hand rail in relation to the direction of travel. When the signal is displaying an indication other than Clear, the inductor is energized and a pick up coil mounted on locomotive or control car would sense the magnetic field and trigger the alarm in the cab.
Although less popular in the east than cab signals, ATS was installed on the New York Central Water Level Route between New York and Chicago, before being later removed by Conrail. The system saw its most extensive use in the Midwestern and Western U.S. For instance, the Chicago and North Western Railway's installed ATS on its Chicago area commuter lines as well as its route from Chicago, Illinois to Wyeville, Wisconsin, though it was later removed from the later in 1964. The ATSF installed ATS on much of its transcontinental main line, which is still used by Amtrak's Southwest Chief as well as its line between Los Algeles and San Diego, which is still used by Amtrak's Pacific Surfliner. The presence of ATS on both lines allows for 90mph operation.
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 (partial) list of ATS systems that are specific to Japan only:
In Wellington only a few signals at a converging junction are fitted with mechanical ATS. All electric trains are fitted.
ATS systems can be mechanical, where an arm is raised on the track to engage a lever on the train to apply the brakes and cut the power. The mechanical systems around the world are generally incompatible. Mechanical systems are not suitable at high speeds, say greater than 110 km/h.
ATS systems can also be non-contact magnetic or inductive. Again they tend to be incompatible. Non contact systems are suitable for high speeds.