Silly question, but if it's that lightweight, why not just put a battery pack in?
The combination of propulsion, air compression, and HVAC would require a tremendous battery capacity. I'm sure it's not impossible, but a combustion engine is certainly justified in this case for practicality and basic levels of historical accuracy. I don't think 99% care exactly which diesel is in there, but I bet at least 30% would care if there wasn't one at all.
You never know with this thing though! They also said we'd never see a Big Boy run again...And if we did it certainly could never run on oil instead of coal.