Compared to the mechanics of chemical batteries, the idea behind rail storage is simple. During periods of low electricity demand, power is dispatched from the nearby grid to pull a chain of weighted train cars uphill.

And there they will sit—losing no power to degradation—until the grid has a period of high power demand. Then they are sent rolling downhill. Their momentum sends electrons back to the grid through a system of regenerative braking that uses the turning power of the wheels to generate electricity.

ARES predicts its operation will cost only half of what a pumped hydro project with similar energy output would require, and should have a much smaller environmental footprint.

This is actually very clever in that it covers up one of the gaping holes of the modern power grid – uneven availability. We flick on the lights, never pausing to think that that energy isn’t usually stored in batteries (that would be prohibitively expensive), but instead is simply shifted around the grid as needed.

Nifty gadgetry by ARES (Advanced Rail Energy Storage).
Quoted text via Scientific American.