Lofstrom has lately been talking about "PowerLoops" - basically underground loops that run for many miles under railroad right-of-ways in order to store energy from windfarms and solar arrays. He seems to indicate that the ROI could be as short as months by buying utility power at night when its cheap and reselling it during the day. Perhaps if the loops were along railroad tracks, they could be used to propel trains? Probably not, but as a power storage method, they can be underground. http://launchloop.com/PowerLoopAbstract