Our train has reached the greatly feared Gorge of Eternal Peril (the gorge is actually the entraceway to the layout room and the upper horizontal swing bridge is open). In normal operation a train would not be able to reach such a precipitous position - opening the bridge actuates a micro-switch that short-circuits about 5 feet of track on either side of the bridge. If you run an engine into the bridge protection block while the bridge is open the DCC booster sees a short circuit and shuts down the track power while emitting an embarassing Beep! Beep! Beep! to let all the other operators know that you've fouled up royally.

Look at the trackplan.

The bridge swings on a pair of heavy duty door hinges but rests on support surfaces while closed because the hinges can not be relied upon to hold the bridge in precise position over a long period of time (I didn't use hinges from a bank vault). Getting everything lined up requires excrutiating care to ensure the hinges are perfectly vertical and the bridge closed supports are perfectly level. It also requires building the bridge in place or you'll never get the hinges to line up properly.

The back of the bridge is two layers of plywood separated by ribs for extra strength. I used a large piece of 3/4" birch plywood outside the door. The hinges mount to this (and the screws extend into the 2x4 studs holding it up. By using a large are of plywood and liberal quantities of screws the structure is quite stable (and hopefully will remain that way!)