Hanging 'round the tracks today, I saw a piece of equipment I'd never seen before: actually two. There was a short train of gondolas filled with bundles of ties, obviously for a tie-replacement project, pulled by a hi-rail big-rig tractor. I'd seen hi-rail pickups, but not big ones like this. With the fore and aft "training wheels", this thing was essentially a 2-6-2 locomotive, with 3 big ol' tires on each side riding on the rails.
The other interesting thing was the tie picker-upper. Not sure exactly what to call it: it straddled a gondola (or more than one--read on), and was otherwise a standard maneuverable loader with a claw to pick up the tie bundles. But the really kewl thing was that this thing could actually "walk" itself from one car to the next, using a cleverly-designed double frame; the two sections of the frame could be independently attached to the side of the car or not, and they slid past each other as well as riding up or down. (The outside anchors on one frame were labeled "W" and the inside ones labeled "C".) When I first saw it it was halfway on one car and halfway on another, and I got to see the guy move it, kind of like a crab, onto one car.
I was going to say that this could be an interesting modeling project, but it sounds more like something you'd expect to find readymade from Kibri or similar. Although maybe you could adapt a Cat?