This bridge was inspired by Bad Cop's Pursuit (set 70802) and the short section of railroad bridge included with that set. When I first saw it, I thought it would make a great play feature for a train bridge that is actually usable by trains.
Here is the result of all that working and reworking: 12 sections of PF / RC train track (It won't work with 9V, sorry!) with 1 two trcak-long section "failure point" plus 2 studs of space to separate the moving from non-moving items and allow the hinge to do it's job.
The track leading up to the "failure point" as I call it, is raised ever so gently at an angle of (at most) 1 1/3 bricks high per 1 section of track. (The angle of ascent / descent depends on which part of track you are on, but for the most part it's consistent.)
The design of the bridge is modular so that you can easily disassemble the bridge for transport. It disassembles into 2 lower ramp sections consisting of 4 tracks each and 2 flat sections placed onto plates with the 1 "failure point" module consisting of a hinged (on one end) track piece in the middle.
The bridge when the track is safe to cross: the pins are inserted and it should be stable. Naturally, a very heavy engine will snap the Technic rods in half, breaking the bridge permanently. Thus you can only use this engine with Small engines like my 2-6-0 + it's consist, (AKA the Lone Ranger train) the My Own Train series engines, or something of comparable weight.
I don't think putting a articulated such as a Union Pacific "Big Boy" or even a non articulated Pennsylvania "T1" would look very good on this bridge, as these (among other large types) are longer than the bridge gap itself.
This is how it works:
Their are two hidden Technic rods under the track that should allow trains to pass by safely overhead. Pull the Technic connector and your bridge collapses. Lift the bridge up and move the rod back in to reset the bridge for the next adventure.
EDIT 8/29/16: Added another length of track to the bridge;s failure point module and revised the pylons of the bridge. LDD file and pictures updated!