I just learned about this today, PG&E was writing about this more than two years ago: PG&E Currents
ARES energy storage technology employs a fleet of electric traction drive shuttle-trains, operating on a closed low-friction automated steel rail network to transport a field of heavy masses between two storage yards at different elevations.
No fossil fuels used
Water not required
No hazardous waste created
No harmful extraction of materials
Clean decommissioning process with no lasting impacts to the natural environment.
Creators Website: ARES North America