Great Western

In reality, in the time period modelled (september 2015 or thereabouts), the real route had overhead wires primarily for the Heathrow Express trains, though they wired up the entirety of Paddington with a view to future proofing and to allow flexibiltiy for the HEX trains to run in to other platforms while they were there. Significantly easier to disrupt the station once than to do it multiple times for something like this.

By around the period modelled in the route, the real line was in the process of being upgraded to provide full electrification along the entirety of the line to Bristol. At the point modelled, it's relatively early days and you see after Airport junction that the gantries are fully installed and then as you proceed over towards Reading they get less and less complete until near reading there's barely just support beams in on the sides of the track let alone anything else.

Of course by now in reality the line is now electrified and electrics run up and down the line all the time - but as modelled, the route primarily was run by diesels, plus the Heathrow trains which were the only electrics if I recall. So while you don't have any electrics to run on the route, it is modelled as per the period so that it looks correct.

Hope that answers your question.
Matt.