I don't have time to look for the cite now, but I've read a proposal to build electric traction railways up some steep slopes in SoCal, then load up cars of rocks, mine tailings, slag, etc. The trains would head up the mountain at night when electricity is in surplus, then back down in the daytime when demand is near peak.

Seems outlandish, but the calculations predicted very high efficiency and comparatively low cost per MWh of storage.