Russell,
Don't over think this bridge thing. It as simple as having a board that lifts up. There are automotive hood/trunk supports and furniture cabinet lid supports that are available to gas assist the lift and hold it when up.
There's a few easy ways to get it done - but they are all the same concept.
here's one example by frizzinbee on his layout - look at the March/April time period.
I have a couple of other styles of hinging bookmarked. They all use a gas cylinder. Some have even used storm door closers, but that seemed clunky and a little crude to me.
One that I liked just made the section terrain the same as the rest of the layout. it doesn't have to be a bridge. They have had lift sections for bars for years. It's not a complicated thing. The gas cylinder takes the weight off the lifting motion and holds it open.
