Basically, the microswitch http://en.wikipedia.org/wiki/Miniature_snap-action_switch will sit on the dropping section, so that when the bridge drops away, the switch loses contact.
The entire bridge section and the previous track segment (outside the walls) are one isolateed section. Power to them runs through the microswitch.
So, if the bridge is open, the circuit is broken and there is no power to the entire section and for 3-4 feet beyond the bridge entrance walls.
Bridge closed, switch closed, circuit is powered.