There a time required for change of signals. So a passing train does not get caught in block with a restricted change.
So if the bridge tender has the authority to open the bridge. He has to set the signal, then "set the clock" and wait "5" minutes before he can start to raise the bridge. This delay prevent a train from getting caught in block and facing a home signal red, or raised bridge, and or a derailer set to put him off the track while the train is still travel at max track speed.
Time of delay would be fix for a location and might be different between site requirements. (Speeds, Line of Sight, how far back the last signal, etc)
Don't forget fail safe is always to stop traffic.
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