There also is this idea: <a href="https:&#x2F;&#x2F;www.ecn.nl&#x2F;news&#x2F;item&#x2F;floating-train-at-2000-kmh-set-to-store-10-of-dutch-electricity&#x2F;" rel="nofollow">https:&#x2F;&#x2F;www.ecn.nl&#x2F;news&#x2F;item&#x2F;floating-train-at-2000-kmh-set-...</a>. It would store 20 GWh in a maglev train riding at 2000 km&#x2F;hour on a 15 km circular track. More or less an enormous flywheel.