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It is true, AC motors run at essentially one speed, determined by the number of electrical poles ususally on the stator, but this is true only for three phase synchronous motors. Straight poly phase induction motor's rpm will vary as the load is increased, difference in RPM called "slip". A commercial DC motor's speed is varied by changing the armature (ususally the rotating part) current, i.e., the magnetic field. Any wave form other than DC (none) is AC, even a lighting strike (fast time rise). AC locomotives generate AC which is rectified to DC and then "reconverted" to AC usually using a PWM (pulse width modulating) system quit similar to DCC systems. The motor RPM is controlled by the width (therefore the number of "pulses/second) of the pulses. The "frequency" of the number of cycles creates the singing noise heard in the earlier DCC systems. To be authentic, you'd need three phase on your "track" to use a straight AC motor (or a convertor in the loco).
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