Assuming that the additional service will be a Barnstaple to Axminster stopper as the report seems to suggest, my guess is that the stock would more than likely to be the equivalent of a 150 assuming that they'd been replaced by the mid 20's
I was picking up on this point in particular:
Provision of an additional stopping service between Exeter St Davids and Axminster would support forecast passenger demand into Exeter in the peak periods as an alternative to substantial train lengthening of the London Waterloo service.
In other words, my suggestion above would provide longer trains on the Waterloo service, rather than an Exeter-Axminster stopper.
I will admit to having a certain bias having grown up near the route but there are a number of potential advantages to electrifying the whole of the Basingstoke-Exeter route over Newbury-Penzance.
1. You release 41 well-maintained class 158s and 159s which can work over various areas of the network - I'm sure Northern, ATW▸ and Great Western could find a use for them. In comparison electrifying the Penzance route releases a large number of HSTs▸ which unfortunately due to their age and suitability for express services only would surely be going to the scrapyard. It also allows all-electric working into Waterloo.
This is essentially what I'm trying to get at, but without the cost of electrfiying the whole route (not that I don't think wiring the route is a good idea, just that there are bigger priorities, not least extending wires along the
GWML▸ to Plymouth (at least) before the IC125s are finally withdrawn). All-electric working into Waterloo I believe was mentioned in Modern Railways as a requirement to maximise frequency, my hypothetical electro-diesel son-of-159 ought to provide that. Great Western I would suggest need their 158 fleet boosting by arround 12 2-car units to lengthen Cardiff-Portsmouth to 4-car units (with the real 3-car set moving to Northern). If
ERTMS▸ can be installed, the Cambrian Lines could be a good home for the orriginal batch of 159s (with their more-powerful engines) which would release ATW's existing 158 fleet for use eleswhere on the franchise.
SWT▸ 's Salisbury-Southampton stoppers could transfer to
FGW▸ (to be worked by 150/165/166 units) and Eastleigh-Romsey would be a small stretch to electrify. The lower-power '159s' and any remaining 158s could be useful at Northern or perhaps the Bristol-Plymouth/Paignton/Penzance axis on
GW▸ .
Electrifying to Exeter only wouldn't be much use without bi-mode trains, which aren't without their doubters. Electrifying to Plymouth only would not go down well west of the Tamar, but with the numerous tunnels and viaducts on the Cornish main line, it probably wouldn't be cheap.
I'm one of the doubters about bi-mode, but in my view there is quite a difference between 125mph trains, which cover a relatively restricted area of the network which can be focused on for electrification, and 90-100mph regional express units, like the 158s/159s, which cover long distances but mix main lines with quiet branches and will not be easy to entirely electrify in the lifetime of the stock.