Under current plans Swindon gets switched on as part of Didcot - Cardiff/Corsham switch on next December.
I did a return trip from London Paddington to Cardiff yesterday, 15/12/17, I have not got positive news I am afraid for London-Cardiff to be live by this time next year. First of all there is still no wiring on Platform 2 at Paddington.
The following is all looking at the up-lines only, from Didcot there is wiring all the way to about MP74 with the exception of the Steventon Gap, there are still some masts missing here near the infamous bridge and the two level crossings, also there are a couple of gaps between Uffingham MP 66 and MP74 and the A/T cable is not complete in quiet a few places west of Didcot.
Talking about the Steventon Bridge, according to the Steventon Oxfordshire Village Web, the Vale of White Horse District Council will make a decision in January or February 2018 about the bridge's fate.
From about MP74 to Swindon station nearly all the masts are now up including Swindon station itself.
Onto to Wootton Bassett, I would say about 80-85% of the steel sticks are up, then the South Wales main line, nothing much until after the M4 bridge which is about MP84, then for the next 10 miles up to just short of the Hullavington passing loops there is wiring all the way, the loops look like they will be wired as masts and support arms are ready for wiring in the loops.
There are more masts and support arms to just short of Alderton Tunnel, after this more piles for a few miles until they they fizzle out until Westerleigh Junction then mores piles and quite a lot of steelwork near Bristol Parkway station.
From Bristol Parkway to the Seven Tunnel I would say about 70% of the masts are up.
Into South Wales and there is nothing from the Welsh side of the tunnel until about 1 mile east of Llanwern, the piles reappear and there are quite a few masts alongside the steel works, the interesting thing that I noticed is that the up relief has masts and with cross arms fitted to single track on the bridge that crosses the main line at the eastern end and a few more masts further west, so it does look like the relief lines may be wired, who knows.
Then again nothing much from the western end of Llanwern, through Newport until a few miles further west to just east of Marshfield where the piles, mostly with masts appear again, this goes onto until a few miles east of Cardiff and then nothing again through to the Welsh Capital.
So as it stands at present, from about MP74 through to Cardiff which is MP145.5 miles there is only 10 miles of wiring completed, plus about another 7-8 miles of solid catenary installed through the various tunnels on route. There is a lot of piling still to be done and of course the masts to go with them so I really cannot see any electric trains running to Cardiff until late 2019 at the earliest at this rate, never mind at the end of next year, Bristol Parkway is a good bet though for end of next year if they sort out the nonsense at Steventon, but I did see somewhere that Network Rail will not start any work on the bridge until spring 2019 even if they got permission tomorrow.