It's tempting to imagine they could all run on electricity, but it's probably hard, as many routes go through hundreds of miles of rugged nothingness. This is quite different from rail transport in Europe.
http://www.geni.org/globalenergy/library/national_energy_gri...
They follow the same valleys around heavy terrain etc. Feel free to superimpose the grid map and the rail map.
So it's a matter of electric locomotives, poles, contact wires and transformers, no small feat but what can you do.
They have been looking into battery electric locomotives which don't make any sense except for switching yards. I read recently that there are electric and diesel locomotives which can use overhead power where available and switch to diesel when it isn't. That has the advantage that they can electrify lines incrementally.