The only one I ever used was one of the through girder bridges from the 70s. It had no floor structure, just ties with plastic between them to make a solid floor. I took the rails out, added some plastic strips to simulate stringers and crossbeams, leaving the old ties/floor in place, and then laid new ties over them. At almost eye level one can't see the underside, or what's under the track, so it works fine for my MILW branch. Cheap, too, since it was just laying around, which is even better.
