The BIGGEST problem with clean energy

I could not see any reference to Flywheel Energy storage which is also part of the energy storage roadmap.  Since these are long life systems (25+ years) and can store MW of power they are useful for storing energy in short bursts such as when electric trains slow down when they arrive in a station, currently the electrical brake energy is dumped into resistor banks.  If a flywheel is used, then this energy can be saved and reused to accelerate the train as it leaves the station onto the next stop.  Flywheel energy is also used in other areas to backup UPS power and there are other activities where a flywheel is useful where energy transfer is required in 30 second intervals.