Any speeding up of services is likely to need to be a long game to obtain.
The Dawlish Avoiding Line (DAL) is likely to be the thing which brings the most speed enhancement which is even vaguely on the table, however the problem with it is that there's limited services which are likely to use it (the Plymouth/Cornwall expresses and maybe the Exeter semi fasts if there's capacity to get them to it). There's certainly no capacity for more London services.
I've suggested in the past that the best way to get it built is to build the Okehampton route to Plymouth. Whilst this would appear counter intuitive key me explain.
With Crossrail 2 built there's scope to run more services out of Waterloo, if you also redouble the WofE line that would allow you to run the existing Exeter services and a new express service with limited stops.
Whilst that would be slower (end to end time) for some within London the journey time would be quicker (especially for Exeter Central) or at least no slower than waiting for the next express. The same would be true for getting to Plymouth via Okehampton.
Add in the faster (and in some cases direct) journey time from places like Portsmouth, Basingstoke, Woking, Southampton, Weymouth, Bournemouth and the like, which could increase demand then there's a good chance that passenger numbers would increase, over time that increase could be reasonably large.
This increase would be added by the increase in frequency between Exeter and Plymouth potentially generating a higher passenger flow between the two cities.
Perhaps to the extent that there's a need for additional services beyond the +1 from Waterloo.
With all that extra demand between Exeter and Plymouth the speed enhancement that the DAL would benefit more people, especially if you could run a new WofE line service which bypassed Exeter St David's (would need a new chord, assuming that is even remotely possible to build) with them also use the DAL (or probably divert the express that way and send the "existing" Exeter service via Okehampton). As that would be 3tph using the DAL (maybe 4tph if you could squeeze in a semi fast between the two cities as well).
Whilst still not a lot of services for a new line, it's still better than what would be the case with only the existing B&H line services.
Otherwise you're likely to be waiting on a HS Wales and West to get you your London trains to use the DAL.
www.realtimetrains.co.uk
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the WoE route needs more double track as it is, without adding demand beyond Exeter. I refuse to get drawn into another discussion about Okehampton, but that budget wuold also buy a fair bit of realignment on the existing route too. Add cash for Exeter resignalling into *any* increase in traffic, no matter the direction.
