Gravitational storage is hard.<p>For perspective: if the U.S. loaded up all 1.5 million of its rail freight cars to 30-ton capacity and sent them 4km up to the top of Mt. Rainier, that would store 494 GWh of energy.<p>That&#x27;s just over an hour&#x27;s worth of our average electricity usage.<p>Gravitational storage is <i>hard</i>.