The throttle in a diesel locomotive has nine positions, idle, plus Run 1 through Run 8. Changing the throttle position changes the power output of the locomotive. Internally, the governor sets the rotational speed of the Diesel engine based on the throttle position, and the electrical control circuits control the voltage sent to the traction motors. The current drawn by the traction motors can be 1,000 amps or more at low speeds.
An electric locomotive functions in a similar fashion. The throttle controls the power, while internal circuits determine how much voltage to send to the traction motors to develop the commanded power setting, thus controlling the power output of the locomotive.
