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The normal method of controlling DC traction motors on units 304-321 was to use resistances, and or tap changer controlled by the cam shaft in the motor coach. There may also have been a form of field divert which controls the back EMF in the motor field windings
Electric locos had a form of tap changer control, this is tappings off the secondary windings of the main transformer. More modern locos class 90/91 use power electronics to control the motors. In the simplist form the control units compares a referance signal generated by the drivers power control handle to the speed of the vehicle. In either case the speed of the motors is controlled by changing the input voltage.
AC motors need electronic control as if you change the voltage of AC current you also change the frequancy. This can cause damage to the motors, so the electronics manage the frequancy to keep a more constant signal.
In real life the power control handle will show power steps, this is only a guide to the driver as the contol equipment is stepless. This is what gives a smoother ride, and has the advantage of preventing wheel slip in poor conditions
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