While electrification of the Valleys platforms is desirable in the long term, I'd guess TfW don't absolutely need the wires through Cardiff Central immediately, as both the planned Flirts and Citylinks can run through the area on battery. I suspect all parties are very keen to keep electrification of this part of the station OUT of the expensive GW main contract, but there's no reason future wires here, and indeed the broader valleys electrification scheme, could not be connected up to the NR main feeder substation. Once the GW project is finished and the dust has settled, TfW could make a future agreement with NR to extend wires through Central, and maybe even on to Barry and elsewhere, but probably using their own more economic methods, materials and their contractors, by then looking for some more work having completed the TfW controlled parts of the Valleys. The beauty of the discontinuous electrification technique is that TfW don't have to complete everything before the new trains can be introduced. Extending wiring further, progressively in the future can still make sense however as it should increase range, resilience and battery life, and with bi-modes diesels, can reduce engine hours (thus extending servicing intervals and reducing maintenance cost). If Amey prove they CAN do cost effective electrification on the Valleys Metro, then it would make a lot of sense for DfT, TfW and NR to allow them to carry on with some more work in South Wales. It is not neccessary for more infrastructure to transfer to direct TfW control to allow this.
