As a railfan, I find these trucks really interesting as well. I've never thought about building one, simply because there are no affordable, nor readily available model trains in a scale that matches the typical Tamiya or Wedico trucks, so one would have to be happy driving it around, off the rails. But in my opinion, it would be kind of like having a wheel loader, but not being able to use it to load trucks. I think you would be missing out on some 'play factor' by not being able to run a hi-rail truck on the rails too. Never the less, they are neat, and I've only ever seen one scale RC build of a hi-rail truck.
http://www.scale4x4rc.org/forums/sho...hlight=hi-rail
That old Kenworth COE is definitely interesting. You don't see those around much, let alone as a hi-rail truck.