I have no idea, but it seems like you should be able to use a direct mechanical coupling to put the braking energy right back in the flywheel, thus avoiding two

I have no idea, but it seems like you should be able to use a direct mechanical coupling to put the braking energy right back in the flywheel, thus avoiding two conversion steps and their associated losses (mechanical -> electricity, then electricity -> mechanical). Whether the savings would be worth the added transmission complexity? Not a clue.