"Flywheels" made me think of an interesting test that was done with city buses in Germany, I think. A giant flywheel was mounted horizontally under the floorboards of the bus, and "charged up" when the bus was in motion.
As the bus pulled away from each stop, it used the stored power of the spinning flywheel to do the hard work of getting the bus up to speed. Which allowed the bus to have a tiny engine (like a 3-cylinder, or something crazy like that!), and still carry a full load of passengers.
Of course, a spinning flywheel has a LOT of potential energy stored, and any little problem can cause them to come apart, and destroy everything around them.
So they developed a flywheel that was entirely spun of fiber (maybe carbon fiber? kevlar? can't recall), so if anything went wrong, the whole thing instantly turned itself into a giant ball of fluff! All the potential energy was released in one massive discombobulation that resulted in a cavity full of cotton candy, rather than pieces of metal flying everywhere in the cabin of the bus.
I assume this didn't work out in the end, because I heard of this maybe 20 years ago? And haven't heard peep of it since then.