First working prototype:http://www.youtube.com/watch?v=7oJIO91q0AA
Second working prototype:
The main purpose behind making the prototypes was to find out how about I'm going to make the thing work (wheels spinning, steering, speed, motion paths, suspension, weight, cogs, trailers).
I went down the manual animation route after first planning on using dynamics after a few talks with Pat Imrie. Things seem to be working OK with work progressing into making the trailers work with the opening sides and massive tree pinging out of it.