Anyway, the history I learned was that a most trains had standardized on large DC commutator motors, which could also be used in AC. Problem was, the higher the frequency, the more internal inductance problems you had. 25 Hz was a good compromise between internal inductive loss and transmission loss on the line.
Actually you have a point. If the DC motor is not a permanent magnet motor, then yes it will run on AC. But then, having the AC be low frequency helps mitigate inductive loss as you mention. AC would also avoid asymmetric corrosion issues that would make apparatus need servicing sooner than otherwise.
I could envision a kind of rotational rectifier apparatus, a synchronous motor driving a commutator, to get pulsating DC.