If you had 6 inch wheels, you would still be able to drive up the rail like this.
http://www.youtube.com/watch?v=3qWYAOGZVM4
Not necessarily recommending it, but it is using small wheels to drive up a large obstacle.
If one was to use Gates belts, would tensioning them be a problem when the barrier pushed into them?
I think high centering could be a large problem. Could this be solved if you had a convex bottom part of your frame so it had a turtling effect (not stable so it doesn't balance when resting on the frame between the wheels)?