The issue with regen braking is that the train dumps a lot of energy as it brakes. The battery (or ultracap) bank serves as a buffer for that. Say you were braking and didn't have trains in the immediate vicinity accelerating - that energy would go to waste unless you wanted to fuck the feeders.

London faces a similar predicament. Due to its antiquated power supply, they had to do some changing up to even support regen braking.

Bear in mind that the "revenue" part is just clever accounting. The idea being that the electricity bill is pretty much constant to the existing and any "sold" energy is icing on the cake. If you wanted to put it into real terms, you can subtract the revenue it from the bill - ergo a smaller bill. Though if you weren't paying for the power, I guess it is free money in a way.

An effort in Tokyo has been in place for a while to collect any spare energy and use it to power the stations.
http://www.mitsubishielectric.com/news/2014/0918.html

>>131552
Interesting. I imagine having the battery banks track side allows to lower the weight of the rolling stock a bit?

>>131554
They don't touch the rolling stock. The regen braking on the stock is the same as before. Although if the system was once using rheo braking then yeah, it would reduce weight as you can dump the resistor grids.