(I remember your handle from previous discussions where we mostly disagree, but I certainly think your current summary of the typical discussion landscape is ve

(I remember your handle from previous discussions where we mostly disagree, but I certainly think your current summary of the typical discussion landscape is very accurate) I often wonder about non-compact kinetic energy storage, like huge fly-wheels or railcars, since kinetic energy is quadratic with velocity. When using a large radius circular railway, the limiting factor is no longer tensile strength as in usual fly-wheels. I wonder how low frictional terms can be brought for evacuated tunnels. the rail could be the stator and the circular train the rotor, or the other way around: a constant cross section "train" of mass with a rail as the rotor, and the fixed train wheels as the stator underneath... if it is underground or in a pit accidental energy release could be less catastrophic.