... to skin an overpass. On mine I used slices of 1x3 lumber in graduated sizes. I attatched a masonite roadbed to the the tops of the slices of wood. I had turns to take into consideration so I cut the masonite wider than the track to accomodate cork turn aprons (worth thinking about width, cuz its tough to add on width after the fact). There's a bunch of schools of thought. The train-boys have lots of intel on overpasses. Look on a few of their choo-choo sites for tips too.
Here's a few not so great pics. Mostly showing the need to plan your width as well as height. nd