It could be hubmotors, and it doesn't have to be electrical, there are motorcycles out there sporting gas powered hub motors, like Batman The Dork Knight, from what I understand that one had TWO which would be insane to keep in tune.
I dunno, though, it could be something more complex. It could just be using something similar to the effects of high temp super conductors using the magnetic leylines in a sort of way like ships use sails to propel itself. Hey, there are brakes that work using opposing magnets without any pads making contact with each other, I suppose they could use the same tech to propel one, too.