Not only that, but the London Underground already uses regenerative braking. Here's the press release from 2013 from when Alstrom got the contract to put in the system that is now being tested, http://www.alstom.com/press-centre/2013/5/alstom-to-supply-g... :
> Alstom was awarded a contract by UK Power Networks Services to supply its innovative Harmonic and Energy Saving Optimiser (HESOP) energy recovery system for the Victoria Line of the London Underground. ...
> London Underground already makes good use of regenerative braking but, by adding HESOP to the power supply arrangements, the residual energy that is currently wasted in braking resistors can be made use of – this will help prevent tunnel temperatures rising.
> Alstom was awarded a contract by UK Power Networks Services to supply its innovative Harmonic and Energy Saving Optimiser (HESOP) energy recovery system for the Victoria Line of the London Underground. ...
> London Underground already makes good use of regenerative braking but, by adding HESOP to the power supply arrangements, the residual energy that is currently wasted in braking resistors can be made use of – this will help prevent tunnel temperatures rising.