Hopefully we're not missing the important point here, which is that completely turning off a locomotive is NOT at all equivalent to turning off the generator field switch. Imagine a switching job in real life, where every time a crew member has to go between to uncouple cars, hook up brake lines, etc. that the engineer first had to say "Hold on, let me shut down all the locomotives... okay, go ahead and uncouple... okay, now give me a few minutes to turn the engines back on again..."
Long story short - there is no generator field switch in ORTS, and turning off the locomotive is not at all a practical substitute for that.