I did a return trip from London Paddington to Bridgend yesterday and I will give an update here but no where near as good as
59CosG95
Apart from a few tiding up bits around Swindon Station, i.e. from temporary to permanent fixings for the contact wire I recon all the wiring is done more or less up to just short of milepost 97, from here to Bristol Parkway there are very few masts missing and nearly all with SPS fitted.
There is also a few lengths of wiring around MP 106 and the return wire is installed in a few more places right up to Westerleigh Junction.
Bristol Parkway to the Severn Tunnel has about 75% of the masts installed, there is no much in the way of masts between the Welsh side of the Seven Tunnel and Newport Station, only about 20%. Again nothing much until you get to the western side of Ebbw Junction, from here to about Wentloodge Freightliner Depot there a very few missing masts and nearly all have SPS fitted, then a few more masts here a there up to Cardiff and three masts installed on Platform 0 at Cardiff Central.
How far west will the wires go you may ask? well on the western edge of Canton Depot there is a new substation, so this I guess this will be the limit.
Switch on to Bristol Parkway by the end of the year is a big Yes, but to Cardiff? no way, I recon it will be spring/summer next year.
I must say that the pointwork just west of Cardiff Station is a complete mess, they have done absolutely nothing to simplify it with the recently re-signalling at the end of 2016.