The speed of a locomotive is controlled by

In electric traction (and specifically steam locomotives), the speed is primarily controlled by adjusting the input power to the engine.

This is done by regulating the flow of steam entering the engine cylinders, which directly controls the force generated and hence the speed.

Flywheel is used for smoothing rotational motion, not for speed control.

Gear box is generally not used in locomotives in the same way as in automobiles for speed variation.

Applying brakes reduces speed but is not a method of controlling or maintaining desired operating speed.

Therefore, the correct method of controlling locomotive speed is regulating steam flow to the engine.