DC traction motor design hasn't differed too much from Sprague's 1882 Richmond, VA, installation. The refinements have been interpoles, which provide for sparkless commutation, and improvements to insulation materials. The "wheelbarrow" mounting is the same: one side of the motor is attached to the axle, the other to the truck frame.

Hollow axles will shunt track circuits just fine. The SP 2472 has hollow driver axles (you can shove a coke can through them). In fact, a hollow axle can be stronger than a solid one.

The Milw bi-polars are the most famous, but they are at least 10 years younger than the NYC S-motors, also bipolars, which outlasted the Milw ones by about 15-20 years. The big drawback with a bipolar traction motor is the significant loss of efficiency caused by the rather large air gap between the field pole pieces and the armature spider. In any motor, the larger the air gap, the less the efficiency.

AC traction motors are essentially the same as DC ones except for the lack of a commutator and the 3 phase field arrangement. The rotating 3 phase fields (the wiring doesn't move, just the magnetic fields) induce "counter currents" in the armature, and these circulate within the armature only. The interaction between the two sets of current causes the armature to rotate.

In a DC motor, the current in an armature coil causes it to try to align itself in the polarity of the field, but since the commutator is a pole-changer, the armature coil's current is reversed just as it lines itself up, which means it now tries to align itself with the next set of fields. When it gets there the process repeats itself, and on and on. Since the coil is embedded in the armature spider, it makes the armature rotate.

The weak point of a DC traction motor is the commutator--it fails rather easily because overheating caues the copper bars to soften to the point where they no longer can be held in place by the V rings. Since AC traction motors have no commutator, you can do all kinds of things, like stall braking, that you can't do with DC. On the other hand, they have poorer torque/speed caracteristics.