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The receiver consists of a two coil electromagnet carefully aligned to pass about 1.5 inches above the surface of the inductor shoe. The inductor shoe consists of two metal plates set into a streamlined housing designed to deflect impacts of debris or misaligned receivers. The metal plates are connected through a [[Choke (electronics)|choke circuit]] in the body of the shoe. When the choke circuit is open magnetic flux in the receiver's primary coil is able to induce a voltage in the receiver's secondary coil which in turn triggers an action in the locomotive. When the circuit is closed the choke eliminates the magnetic field and the voltage induced by it allowing the locomotive to pass without activation. Where unconditional activation was desired specially shaped metal plates could be used in place of a fully functional shoe, however the design of the system can result in [[false positive|accidental activations]] when the train passes over switches or other metal objects in the track area.
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">{{Cite web|url=http://www.railpac.org/2008/10/02/a-look-at-automatic-train-stop-ats/|title=A look at Automatic Train Stop (ATS) – RailPAC|date=2 October 2008 }}</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. The receivers can also be designed for easy removal to prevent damage when operating in non-equipped territory or to cut costs when only a small portion of the railroad requires ATS equipped locomotives. "Inert" inductors are sometimes placed in advance of certain speed restrictions as an alert or at engine terminals to test the functionality of the ATS system.
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