I wondered if the best thing that could be done for the southwestern line from salisbury would be to straighten out some of the bends rather than simply double tracking it on the current alignment. It is very twisty in its western sections.
I would also have liked to have sen some electrification!
It could well be, although you may need to have some of the existing line retained if such a straightening was to result in a bypass to one of the more minor stations.
Post Crossrail 2 the ability to run 2tph through to Plymouth (one acting as a slightly slower service out to Basingstoke before then running fairly fast to Plymouth with 7 or 8 stops to Plymouth, whilst the other with the same calling pattern as the Exeter services today before all stops to Plymouth) could be quite attractive in picking up more inter regional (i.e. not London) passengers.
For the extra path it could be Waterloo, Clapham Junction, Wimbledon, Woking, Farnborough Main, Basingstoke, Salisbury, Yeovil Junction, Honiton, Exeter Central, Exeter St. David's, Okehampton, Tavistock, Plymouth.
Whilst that allows it down by calling at Wimbledon and Farnborough before Basingstoke, both these locations are key locations. Wimbledon due to its connectivity to a wider area of London which could put journey times at least on a par with the GWR route, whilst Farnborough is an urban area with a population only marginally smaller than Plymouth and with its current good connections to Reading on the NDL already is likely to see a fair number of passengers heading towards the West Country.
With ~9 stops fewer than the current service from most stations between Yeovil and Waterloo that's likely to cut the journey time by around 20-30 minutes. Electrification to Salisbury, or better still Yeovil, would ensure that it's towards the upper range. 100mph running to Basingstoke and 110mph running over sections between Basingstoke and Plymouth would likely cut journey times further.
Whilst probably not enough to beat the London Plymouth times on the GWR services, enough that as long as the times were staggered from the GWR service that it was an option that allowed a later departure than the proceeding faster train and an earlier arrival than the subsequent faster train.
However the main focus of such a service would be to pick up passengers from the areas which have good connections to Clapham Junction and Wimbledon as well as those areas South of a line between Woking and Yeovil and possibly as far East as Brighton to connect to the West Country much more easily than is currently the case.
Even providing a comparable Exeter to Plymouth journey time would attract more passengers due to the extra frequency, especially as there's no need to change when traveling from Exeter Central, Cranbrook (for Exeter Airport) and Honiton.
Add an extra call on the existing services at St. James Park and it would improve connectivity between more of Exeter and Plymouth even further.
It should be noted than when looking at this is likely to be circa 2040 and so it should be viewed through the lens of the extra passengers which are likely to be using rail services at that time. As well as the need to be rapidly reducing our carbon emissions and so reducing our car use.
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