pivot bridge

As my first lawout gets underway - all benchwork is done and being moved in to position this week - now, of course, a change of plans or thoughts, would like to build a bridge that pivots on its center. Any thoughts or ideas on the motorizing and stopping at 90 degrees then pivots back - of course the tracks will need to line up.
thnks for ther thoughts

Walthers makes a swing bridge and its on sale right now.
http://www.walthers.com/exec/productinfo/933-3088

Bryan