That's right! According to Bob's book, the best place to put the huge natural gas cylinders was between the wheels for traction and that's why the tractors have such big "shoulders". He stuck a big block Ford engine in it and used the front axle from a Chevy van. Later, when they were switched to diesel engines the bodies weren't re-styled so even the modern tractors at DLR "have no idea why they're so big and strange".
Regarding the trams themselves: Enjoy this entertaining YouTube video
Then check out this forum thread to see how to build you own!
http://www.pirate4x4.com/forum/general-chit-chat/537959-offroad-wagon-tech.html (about halfway down the page)