Mike, I think that as we all get longer in the tooth that building track would win out over moving dirt. I suggest a dump body as we could run the car right to end of track and Casey or Kubota could put the pile right into the hole. There will be plenty of shoveling opportunities to spread and compact the backfill. My idea would be to use at least two axles under a frame with a pickup truck bed lifted by either a hydraulic or pneumatic cylinder. Maybe a couple of the Koppel axle/frame sets would work for the wheelsets.
Ira, I totally agree with the idea of building a Portland company hopper not just for the practicality but since none were ever built we would have a one-of-a-kind Portland car. I wouldn't use it for fill though, to unload we end up dropping everything between the rails, then have to shovel that out and still have to move by hand. Too much extra work. With adjustable gates, perfect for ballast spreading.