Thinking wildly, tube trains are like pistons (they're actual plungers, I guess) if you had a smooth walled section of tunnel then you could have a seal at the head of the train that fans out and provides pneumatic braking, you could maybe use the pressure to drive the ventilation system. Unlikely to be practical (you'd have to have a pre-platform door to prevent all the passengers from getting exposed to the pressure increase, I think).
Another approach that can be used when initially building the system is to have a slope leading into/out of stations so that energy can be stored as gravitional potential. I know the Montreal Metro was built this way.
Actually regenerative braking is common on electric trains. Stock bought since at least the 1990s for London Underground has regenerative braking of various forms, with the more modern being fitted to S-stock (the sub-surface lines which were all gradually replaced with a single type of train, if you went on a "Tube" train and it was actually walk-through the length of the train and seemed vaguely modern inside, that was S-stock and you weren't in the deep tube where heat is a bigger problem)
In the 1970s when the oldest stock still in use was purchased, regenerative braking was not a thing.
Some old freight routes used electric haulage because the regenerative braking going downhill almost completely offset the electricity consumption going uphill.
Or if the braking resistors have to be close the the coils, then you could use a heat-exchanger to move the heat somewhere else...
the more elegant solution in use around in the world is to put the stations on hills
Well that's horrifying. Is there no health and safety monitoring being conducted?