There's a video of Charlie Comstock's swing bridge and how it works. I suspect that it will give you some basic information, and I'm willing to bet he would be willing to answer any questions you have.

these:

http://model-railroad-hobbyist.com/node/5909

http://model-railroad-hobbyist.com/node/6017

http://model-railroad-hobbyist.com/node/1683

http://model-railroad-hobbyist.com/node/5539

(There may be others, so you might want to search for them using the search box in the upper right corner.)

He (JLandT Railroad) documented his troubles and tribulations on his lift bridge. It's complete with pictures, drawings and video

Steve

Unintentionally running a train into an empty gap is much more difficult if both side of the gap lift up. London's Tower bridge for example.

While this particular example is a road bridge, you can see that the opening is blocked to traffic when the bridge is up.  It will be a more complicated device to build, but comes with much more peace of mind!

If you build a bridge that opens in the middle, it won't have any support in the center.  That means you need to engineer it well enough that there is no "sag" in the center when the bridge is down, and that the rails will always line up perfectly when the bridge is lowered into the operating position.

You can accomplish the same thing in terms of stopping trains from falling when the bridge is open by installing a switch to kill power on both sides of the bridge when it is open.  The section of track that is killed on each side must be longer than any of your flywheel equipped locos will roll after power is shut off.  My P2k E-8s and Pa's coast quite a ways at passenger train speeds when the power is shut off.