Interesting idea here from Tom Bartley : http://www.cleanfleetreport.com/rail/clean-energy-freight-trains-gravity/
At first read I thought this idea was too expensive to implement to be practical.
Then, remembering my drives through Helper, UT, I saw the possibilities. Helper sits at the base of a steep pass that is very highly trafficked with coal trains destined for points east. The pass is so steep than up that up to 12 locomotives have to be added to trains just to make the crossing. Interestingly, a small coal fired power plant feeds the grid from that pass.
It would be quite practical to install the (15 miles?) of conductors needed for electric locomotives (well they are all electric, but you get my point), the power grid is in place, the local power plant can provide the uphill energy and the grid captures the downhill energy. No batteries required, and the special locomotives would only be added for the trip up and down the pass, as they are now.