Putting a dump truck on a flat car (or even a speical car) has been used at other places. This seems only works when the distance to travel is short and there is no other way of getting the rock in to the site. Other difficulties involve getting the truck up on the flat car; that is why I recall once seeing a picture of a very low flow car being used by a contractor.
Another problem is the quanity of rock moved in a dump truck. From my experience it takes two deliveries by a double-bottom dump truck rig to fill up a standard gauge 35'ballast hopper.
Brian Norden