The London Underground uses 630 VDC traction power. Power distribution is 11KV/22KV 50Hz, converted to DC at substations every few stations along the right of way.[1] 630V is too low a voltage for long distance transmission without big losses, so distribution is at a higher voltage. AC to DC conversion is with rectifiers, which are one-way; they can't make AC from DC. With inverters, that's possible. Inverters are basically big switching power supplies. This is just an inverter installation at a substation. It's probably bi-directional, converting AC to DC or DC to AC as required.
The whole transit system isn't one big DC circuit; it's in sections of a mile or so. Regenerative braking can only power trains in the same section, unless traction power can be up-converted back into the AC system. If more trains in a section are braking than accelerating, the excess power has to be dumped somewhere, usually into big iron resistance grids that waste it as heat. Using inverters eliminates that energy dump.
The whole transit system isn't one big DC circuit; it's in sections of a mile or so. Regenerative braking can only power trains in the same section, unless traction power can be up-converted back into the AC system. If more trains in a section are braking than accelerating, the excess power has to be dumped somewhere, usually into big iron resistance grids that waste it as heat. Using inverters eliminates that energy dump.
[1] http://www.trainweb.org/tubeprune/tractioncurr.htm