Complex sentence: The speed of the locomotive is regulated by pulse width modulation, that is the track current is switched on and off rapidly (hundreds of times per second) and the time for which the current is switched on is proportional to the speed step.
Split into simpler sentences: The speed of the locomotive is regulated by pulse width modulation. That is the track current is switched on and off rapidly and the time for which the current is switched on is proportional to the speed step.