It depends on the type of engine tbh Styles, lack of power to the regional grid makes electric trains a liability.
Steam requires a lot of water and fuel, disel or bio ethenol requires either the base oil or crops.
Science based fussion / fission ones could work, if the NCR's OSI could figure it out and actually send enough men to get one running.
My view the trains are just mass sleepers being driven by brahmin packs on tracks.. cheap, effective and requires little work to maintain.
Mentioning Brahmin makes a good point. Can make bio disel with left over brahmin parts and other bio- fuel from crap.
Still one train, one driver a few mechanics to move thousands of troops is way cheaper then. Trucks which you'll need a driver for each one. Mechanics to rebuild and maintain those trucks. The Fuel for those trucks, parts, the infrastructure needed to move thousands of troops.
Its not really a matter of can they do it? Its a matter of which is cheaper and easier to do.
Also nuclear power is not really an option. World running low on uranium before the war.
I hope the Engine of FO4 allows for wheels because FO1 and FO2 have large Caravans of Brahmin pulling carts made out of old cars. I want to see that again insted of brahmin carring everying.