I think the average traction motor is too large in height to be mounted entirely above the axle. Therefore, a significant part of it extends below; from what I have seen, it probably averages eight or ten inches below the centerline of the axle.
10-4 on the mammogram there. Some people, for various anatomical reasons, stand tall when they lie down.
Again, not familiar with transit/subway motor car stuff (except that, no they aren't like Lionel ;-) but at least in NYC the third rail, whether Transit or NYC, er, Metro North all have the rail on the outside. There may be someplace somewhere that does it differently..
On freight railroads, your typical diesel electric locomotive is on a 42" diameter wheel (ref: GE AC6000) or there-abouts - GP38's are 40" I believe - so that puts the center of the axles at a nominal 20" above the rail head. Typical rail height is in the area of 7" (more than you ever wanted to know about rail dimensions) so the maximum clearance above the ties would be 27" or so. The traction motors do indeed extend below centerline of the drive axle, but exact dimensions I don't know. But.. here is a page from Progress Rail that shows the typical truck assembly so you can decide whether you want to check out whether you would fit under one of these.
As I said, I decline to be the test body... ;-) And Happy New Year!