http://www.epanorama.net/newepa/2016/05/22/the-worlds-largest-coal-company-is-going-solar/
http://futurism.com/worlds-largest-coal-company-going-solar/

Startup will build a better grid by pushing heavy railcars up a hill.

One of the biggest challenges of the shift to renewable energy like solar and wind is energy storage. The wind or sun don’t always generate power exactly when it’s needed, so infrastructure needs to bridge the gap. Batteries are the most frequently discussed solution, but there are other options out there, many of them surprisingly low-tech.

In April, the Nevada Bureau of Land Management granted environmental approval and a land lease to Advanced Rail Energy Storage (ARES), a startup with an energy storage solution that’s both novel and old-school.

ARES proposes to use excess off-peak energy to push a heavily-loaded train up a grade. Then, when the grid needs that energy back, the cars will be rolled back down the slope

ARES’ solution is related to an already common kind of energy storage known as pumped-storage hydropower, which pumps water uphill, then captures the power of its downhill flow as needed. The obvious advantage of the ARES approach is that it’s more adaptable, without the need for lots of water. ARES has also said its solution costs about half as much as other storage technologies, and claims 80% efficiency in energy reclamation, similar to or slightly above typical hydro-storage efficiency.