If the railroads were to make motive power decisions based on pure economics, the priority of motive power/fuel sources would be as following, cheapest to most expensive, based on today's price projections:

1. Coal-fired steam locomotives (using ROM coal)
2. Steam locomotives fired by coal-water fuel (aka Silverado Green Fuels)
3. Diesel locomotives fueled by coal-derived synthetic diesel
4. Diesel-electric hybrids (aka in the mode of NH's FL9's), with electrification limited to chokepoints (aka long tunnels as enumerated by Ernest P Robl)
5. Limited electrification of chokepoints with separate electric and diesel locomotives - electrics would probably be operated by remote control through the chokepoints to reduce labor costs of redundant locomotion
6. Comprehensive electrification of most mainline trackage - probably operated per Milwaukee's electric/diesel lashups

I do know Railway Age Magazine suggested using railroad ROW's to host high tension transmission lines, then have the railroads feed off these lines (e.g. the railroads would then be able to forego having to put up the expense of electrification capitalization), but I believe there are inherent safety/regulator aspects that would negate that idea - kind of like putting a small nick-nack shop next to a high speed rail line hosting out of control Pennsy T1's!

But since it isn't free market economics that is driving such decisions, but rather government intervention coupled with the inherent natural monopoly of integrated private railroads, what actualy happens is anyones guess.