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If you have a DC supply for a motor being controlled by a SCR or thyristor then switching the device off would have no effect until the current through it falls to a low value (Ihold). This would occur naturally if the DC supply was unfiltered since the voltage would fall to zero between every half cycle.
If you connected a second device across the first via a capacitor, triggering the second device would cause the current to be diverted to it via the capacitor so the first device would switch off, once the cap was charged the second device would switch off. This is forced commutation. Its complicated with multiple drive pulses, but in its time it was the only way of controlling the current in large traction motors.
For high power work , like train electric motors, the overhead supply is 22KV AC from a transformer. So in the locomotive, there is a rectifier, so while the voltage is DC, there is no smoothing on it as the components would be huge and expensive and unnecessary, so DC is pulsing.
Have you looked at :- https://en.wikipedia.org/wiki/Traction_control_system The site has a search function so you can search it for any terms you are unfamiliar with.