It looks like Barcelona is setting the standard for regenerative braking.

"One-third of that electricity is used to help power the trains. The rest provides electricity to power to amenities in the subway stations amenities and energize a growing network of EV chargers."

"has installed three inverters so far, and 13 more are in progress. Once they are all in place, it expects regenerative braking to provide 41% of the energy needed to power the trains, a renewable source of energy it says will save about 3.9 metric tons of CO2 emissions annually."

"Since implementing regenerative braking, the temperature in Barcelona’s subway system has decreased by 1.8 degrees Fahrenheit."