Re: Regenerative braking.
I read in Popular Mechanics several years ago that flywheels are being looked at seriously as a more efficient way of storing electrical power in subway trains. They would tap into electrical rails at each station and get up to speed. They would store enough energy to get the train to the next station. It was suggested that this is more efficient than current methods (I don't know what they currently use). Trains would use regenerative braking to conserve energy, for example if the train had to stop between stations for maintenance crews or a red light, so that it could still accelerate and get to the next station. Seems reasonable to me.