Partial electrification is now being considered, and is disscussed in more detail in this very lengthy thread on a rail forum
http://www.firstgreatwestern.info/coffe ... pic=5066.0
It remains to be seen what trains will be used, but class 319 multiple units (ex thameslink) are likely for the suburban parts, these have the advantage of being cheap as they already exist, but are from reliable, esp in adverse weather.
They are 4 car, and whilst they can be used in multiple for 8 or 12 car trains, this appears unlikely in practice.
For the longer distance parts it appears regreatably likely that something based on the "succesful" Virgin voyager trains will be used, but with duel diesel/electric power.
Such trains have a poor record of reliability, have very limited luggage space, mainly bus seats, no restaurant, and are shorter than existing trains. Not a step forward !
The original idea was for proper intercity trains, with a choice of electric or diesel power.
"Installers and owners of emergency diesels must assume that they will have to run for a week or more"