I'll have a cheeky fiver with anyone that 319s will not come to the Thames Valley routes for anything other than a short stop-gap period, or possibly as a small fleet to work the branches after Crossrail.
I agree that in the longer term
EMUs▸ with a better performance than the 319s will be needed to maximise track capacity. However, it was reported several months ago, after the franchise was extended, that fGW were told by the
DfT» to stop work on procuring such trains. My impression then was that we were were going to be lumbered with the things[1] but I've since changed my mind. It may well be that the forthcoming Direct Award to fGW to continue operating the Western will permit, encourage or require that fGW starts procurement of more suitable trains. My reasoning is that the franchise extension is a continuation of the
status quo, whereas the Direct Award is a new contract with different T&Cs and such requirements can be included.
If the new Direct Award franchisee is as fast off the mark as the new Scotrail franchisee in ordering new trains there might be a fighting chance of getting new outer suburban EMUs into service at the same time, or shortly after, the 800s start running in 2017.
[1] This is unfair - but although they have a 100mph top speed that still leaves a 25mph speed differential between them and the
HSTs▸ and Class 800s. In view of the greater acceleration of the 800s compared to the HSTs, that means that using 319s will cost at least one and probably nearer two paths between Reading and Paddington (and vice versa) on the Mains. Medium term, i.e., post Class 800 introduction, outer suburban EMUs with more oomph and swoosh will be needed.