I would like to make my train overpass a road. What would be a good way to do so?
Once you have your grades figured out so that you have the clearance minimums you’ll need between whatever you place over the bottom track, you should do two things. Figure out how to anchor what you’re going to place there (abutments?), and then fashion an overpass-type concrete bridge. This, of course, is not so much a “should” but is a suggestion.
You could place the grade to the place where the crossover has to happen, fashion the abutments on either side, and then extend the grade on the other side for a way. Then, make a mold out of popsicle sticks, and pour plaster of paris, but Hydrocal would be better…stronger. When it dries, if you have made the mold reasonably realistically, you should have a slab that resembles an overpass bridge. It will be long enough, if you were careful, to span between the abutments, but if too long, sand it on one end. You can use brass tubing or wire to make railings, or pour more Hydrocal to make concrete retainers to place on the top sides of the bridge.
Is that what you were looking for?
A quick and simple overpass can be done by using Central Valley plate girders directly attached to a strip of your roadbed. This will be a ballasted deck bridge. The abutments can be cut out of 1x pine and glue and screwed inplace, or if doing foam just cut and shape the abutments. These can be painted and weatered or can be a base for glueing plaster castings to simulate concrete,or stone.