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== Overview ==
[[Image:Coaster_2103-ATS-inductor.jpg|thumb|IIATS pickup on the leading truck of a [[Coaster (San Diego)|San Diego Coaster]] [[F40PH]]]]
The technology works by having the state of a track mounted "shoe" read by a receiver 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.<ref name="look at" /> The system is binary with the shoe presenting either an "on" or "off" state to the receiver. In order to be [[failsafe]] when the shoe is energized it presents an "off" state to the receiver, 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
The receiver consists of a two coil electromagnet carefully adjusted 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 misadjusted 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">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.
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