This video explains and demonstrates what looks to be a very encouraging development in the storage of renewable intermittent energy, which is one of the greatest challenges facing us today in our desire to transition to renewables.
Just like hydro power uses gravity to make energy, this rail car, a box full of rocks, and the power of gravity can be used to store energy from renewable power such as wind and solar, which could help stabilize the power grid.
This system, called Advanced Rail Energy Storage (ARES), is being tested right now in Tehachapi, California. It uses a diesel engine and carries a four ton concrete pad. Its developers have patented the system and it has been peer reviewed.