Diesel-electric loco's gave locomotive design engineers two really neat features:

- series-wound DC motors have a transfer curve that gives them "infinite" torque into a locked rotor start condition if you can supply infinite current. In reality, the motor drive systems in the early loco's had current limiting circuitry so that the motor would not burn up. But the way a series-wound motor is set up, you get the highest possible torque for the energy into a stalled load. It's perfect for getting a locomotive off the stop.

- a series-wound DC motor could be used to create either dynamic or regenerative braking, something that opened up all sorts of possibilities.

Before electric motors, steam loco's had steam or vacuum brakes, which were nowhere near as good as dynamic braking with motors. The other thing that steam locomotives would do is throw the locomotive into reverse.