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