There are many advantages that AC traction motors have over DC. About one third of a DC traction motor's volume was taken up by that "electro-mechanical switch", better known as a commutator. Commuter DC traction motors had a lot of issues, including carbon brush life, unstable commutators that became oval or irregularly shaped with age and abuse, raised bars, and a tendency to flashovers.
With AC, ALL of that "available" commutator volume was used for active core material and the function of the commutator was basically moved "upstairs" to the inverter(s). AC traction motors are also larger in diameter, and the old rule of "d squared L", applies, where an increase in diameter is much more effective in improving rating than an increase in core length, with its increased temperature gradient (that requires cooling). The "standard" wheel diameter of a USA freight diesel equipped with a DC traction system was/is 40-inches. The most common wheel diameter of an AC equipped diesel is 42-45 inches, and this does permit a nominal increase in rotor diameter.
The tendency of an AC traction motor to run at the specific frequency instructed by the control system has resulted in adhesion improvements that in the DC age were literally laboratory numbers. And that is one reason why modern AC traction locomotives of North American design are rated at a starting tractive effort north of 180,000 lb. I believe (from memory) that MCB type couplers were rated at a yield of 560,000 lb, so you can see that three modern AC locomotives are a good match for a train equipped with these couplers. The corollary here is that with three AC's on the head end, the train WILL MOVE, either as a train or the crew might find out where the weak link is!