With the rise of renewable energy generation in much of the world there has been a need to compensate for their intermittency.
The leading energy storage technologies of today are batteries and pumped hydro storage. Pumped hydro storage is the dominant energy storage system in the world currently.
An alternative being developed in the USA is Advanced Rail Energy Storage (ARES).
Instead of pumping water up and down a hill, ARES drives trains up and down a hill.
They claim the system is cheaper to develop and deploy than pumped hydro, doesn't require water and has an efficiency of up to 78%.
My thought is that there would be more maintenance cost to ARES than pumped hydro, through wear and tear of the rails and carriages, etc.