In steam engine terms from the olden days my new powerchair (the Jazzy) would be known as a 2-2-2 engine. Because it has six wheels but they don't all help to drive it. A steam train/wheelchair like a manual where all four wheels do the work is known as a 0-4-0 engine.
Bet you really wanted to know that, I know I did. But you see you learn these ever so useful and interesting facts when you randomly bump into a trainspotting mad friend of the family at the corner shop!