Because the real McKeen Cars almost never hauled more than one trailer, ever. So I don't need any more then the power to get it up a fair grade. Also, it would be a lot harder to power the back truck rather then the front. I have a exhaust pipe which was such a tight fit in between the wheels, that to make the 036 curve, I had to pinch the pipe in able to make the turn. I don't think I can power the back wheels.