That’s the claim, anyway:
It’s basically a giant battery. Electric locomotives haul massive concrete slabs up a shallow hill, using grid power when there is excess capacity. When there is greater need for electricity, the locomotives coast downhill, using regenerative braking like an electric car. Should be simple and rugged… rails, rail cars, locomotives and concrete slabs are well understood and not exactly delicate.
This would of course be most useful when applied to fickle weather-power systems like wind and solar.