I got the inspiration from a picture of the LEGO display at LEGO World. A LEGO designer took the 4203 set and put it on rails. I don't think he/she used a train motor since the wheels are spaced is much further apart. He/she also used a box car. I think a gondola would make more sense for an excavator to dump material into.
Hi-rail Excavator by dr_spock_888, on FlickrHere it is running on some tracks:
http://youtu.be/EYKANypmtEQ
More photos on my BrickShelf:
http://www.brickshelf.com/cgi-bin/gallery.cgi?f=553098

