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The technology works by having the state of a track mounted "shoe" read by a sensor mounted to a [[bogie|truck]] on the leading locomotive or car. In the standard implementation the shoe is mounted to the [[railroad ties|ties]] a few inches outside the right hand running rail, although in theory the shoe could be mounted anywhere on the ties. The system is binary with the shoe presenting either an "on" or "off" state to the sensor. In order to be [[failsafe]] when the shoe is energized it presents an "off" state to the sensor, while the non-energized state presents an "on" state which triggers an action. This allows things like permanent speed restrictions or other hazards to be protected by non-active devices. In this case non-powered shoes may be employed or specially shaped pieces of steel in place of a fully functional shoe.
The most common use case for the ATS system was to alert the [[railroad engineer]] of an impending hazard and if the alert was not acknowledged, stop the train by means of a full service application of the [[Railway brake|brakes]]. When attached to signals the shoe would be energized when the signal was displaying a "Clear" indication. Any other signal indication would de-energize the shoe and trigger an alarm in the cab. If the engineer did not cancel the alarm within 5-8 seconds a penalty brake application would be initiated and could not be reset until the train came to a complete stop.<ref name="look at">http://www.railpac.org/2008/10/02/a-look-at-automatic-train-stop-ats/</ref> Unlike mechanical train stops or other train stop systems, IIATS was not generally used to automatically stop a train if it [[Signal passed at danger|passed a Stop signal]] and in practice could not be used for this purpose as the shoes were placed only a few feet from the signal they protected and would not present sufficient [[braking distance]] for the train to stop.
On Bi-directionally signaled lines two "shoes" would be needed, one for each direction of travel as locomotives would only have a sensor to detect the shoes on one side of the train. Sensors can also be designed for easy removal to prevent damage when operating in non-equipped territory or to cut costs when only only a small amount of railroad requires ATS equipped locomotives. "Inert" inductors could be placed in advance of certain speed restrictions or at engine terminals to test the functionality of the ATS system. On a few [[light rail]] lines IIATS has been employed in a manner similar to mechanical train stops, stopping the train if it passes an absolute stop signal. It is useful where light rail shares tracks with mainline railroad trains as mechanical trips may be damaged by or interfere with freight operations and because [[light rail vehicle]]s can be bought to a stop much more quickly than a mainline railroad train without requiring complex [[Overlap (railway signalling)|signal overlaps]]
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