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 {TAIL ~ the crossing} is triggered by {HEAD ~ the passing} of the first vehicle through the crossing releasing 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 { TAIL ~ the crossing } is triggered by { HEAD ~ the passing } of the first vehicle through the crossing releasing the interlock circuit
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH