@glynmoody We've been doing this for a very very long time indeed. Very old technology. The exciting bit is what happens when your regenerative braking is generating vast amounts of power and you briefly lose contact with the wires.
Some bits of the underground also have gravity based braking from before even this - there is a rise into station platforms and a fall the other end so that some of the kinetic energy is captured in the rise in height and then turned back the other way as you leave