An interlock circuit is adapted to be activated by the first sensor to prevent a change in the signal light output until the third sensor at the crossing is triggered by the passing of the first vehicle through {HEAD ~ the crossing} releasing {TAIL ~ the interlock circuit} 
An interlock circuit is adapted to be activated by the first sensor to prevent a change in the signal light output until the third sensor at the crossing is triggered by the passing of the first vehicle through { HEAD ~ the crossing } releasing { TAIL ~ the interlock circuit }
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