I don't know where you live but here the clearance on a standard overpass is 14'. This is well under the 12'8 needed for the highway, plus the height of the track base, plus the clearance for the train. So the train would have to be built 30' in the air. Might as well put it above a road instead.

If you mean "put train on ground in the middle of the highway" we can also do that the hard part is getting to stations not in the middle of a highway.