Could the regeneration be putting out a higher current than what the batteries on the subway train can handle? Not very likely, but I guess that's a possibility.
Or if the trains are running on an electrified rail and don't actually carry much battery capacity themselves so regenerative braking fills that small battery and then dumps the rest back to the grid?
I don't particularly feel that either of those situations are likely to be accurate, but I guess someone could have done something weird in the design.
Why do the worst games produce the most amazing reviews?