ARES (Advanced Rail Energy Storage) has been trying to commercialize purpose-built facilities of this type, with the first currently under construction in Nevada. They've gone through a number of design revisions but what they're building right now seems just just be multiple-unit trains full of gravel on straight lines. They've also proposed a system of autonomous trains loading and unloading concrete blocks via switchyards, but I imagine they are holding off on this more complex design for a larger installation, and the current site is a gravel quarry so they're using native materials. Advertised efficiency is over 90% at 5MW per rail line.
