On Sun, 23 Aug 2020 18:23:40 -0700 (PDT), wrote:

All is fine until there is an accident. The flywheel dislodges and

continues like a much more energetic panjanderam, demolishing
everyone
and everything in its very long path.

Yes, though that's mostly solvable. 2 counterrotating flywheels in one
casing with a mechanism to jam them together on impact.

Er, Where does all the kinetic energy go? OK in theory you have equal
an opposite amounts so they "cancel out". But two identical cars
traveling at the same speed in opposite directions into each other
end up a right mess disspiating their kinetic energy,

I think you'd get a big BANG, the desruction of the flywheels and
generation of lots of high speed shrapnel. The latter might be
possible to contain, they manage it with blade failures on jet
engines.

--
Cheers
Dave.