Room 101 - Steam, Diesel or Electric trains?

Typical steam locomotives were less than 10% efficient. The relatively low boiler pressures and no condensors were the main limitation. Some experiments were made with higher steam pressures but they were difficult to fit within the loading gauge. The much higher cylinder pressures in a diesel engine allow efficiencies approaching 40%. 

I would have to put steam in Room 101 due to low efficiency, high pollution and interesting problems operating in sub zero temperatures. How do you ensure a water supply?

With opinionated tongue in cheek…….

Diesel, regardless. They're just dull, noisy and an eco-disaster-zone.
Keep steam because it's fun, nicely sonorous and low enough quantities to not be a significant eco-threat (we could always heat the water some other way - see Electric below).
Day-to-day use electric because it's quiet, clean and efficient (but quite dull). Internal generation from fuel cell or maybe fission (in 20 years of course) because cables suck aesthetically, reliabilityally and infrastructurally.

Bingo! Isn't the above exactly it? An electrically heated steam engine. Wind the pantograph out of the funnel!

Chuff chuff! 

The Swiss did this during WW2 to save coal in their shunters (the main lines were electric). The pantograph was mounted on the cab.