Well, if you can stomach the food, the McDonald's at the east end of Rawlins is pretty good - playland for the kids, UP Overland Route for me. Across the street is a restaurant that keeps changing names, but it has an even better view of the tracks (better food, too, in almost all ownerships).

Denny's at the west end of Cheyenne. Not all tables have a track view, but the ones that do see a lot of action.

These aren't up to the others in terms of food quality, though...