I don't have any ridership figures - how much could we absorb before we'd need more services rather than just faster ones? a chunk of the service is currently short XC Voyagers too, so there's room for improvement via Bristol without adding more paths. Need to point out again that the issue here is that west of Exeter is *slow* - extra capacity via SWR is not going to do anything for that, nor is extra capacity via Okehampton.
Given that pre 80x's the HST's were needed to be upgraded to provide extra seats and that there was much made about the extra 18% of seats to the 80x's it's likely that the loadings to at least Exeter were decent.
Something which I keep reminding people of is that this is likely to be 20 years away. Even if it takes 10 years to get back to 2019 levels of passengers and there's then only 1.25% growth year on year for the next 10 years and that's going to nearly wipe out the 18% extra seats.
However if we see a lot of people moving West to WFH and them then traveling to London (or other South Eastern areas as there's a lot of companies located a bit out of London so they can access London but without needing to pay the high London costs) for work infrequently growth rates could be fairly generous.
In that case it could well be that growth could be enough that the Exeter semi fasts end up well loaded, the 80x's go to 9 coach units (by lengthening 5 coach units to increase the numbers available to add a few extra services a day) and then even 10 coach units all (to increase overall capacity).
Whilst I fully agree that West of Exeter is slow, spending £1.5bn plus on the DAL isn't going to gain a lot (IIRC 15 minutes at best) which would probably see limited extra passengers on a service which would still be hourly (maybe 2tph, but then that requires using the Exeter semi fasts which are slower).
Going via Bristol still requires capacity through Reading, which is limited, and if you're changing off a XC service that's slowing down the journey time and so not really going to gain you much over the SWR route.
Where would the capacity issues in Exeter be? the station itself or somewhere else? if it's the station then how is it going to handle extra SWR services? ( and irrespective of any extra services to Plymouth that needs to be resolved anyway ).
1tph between Exeter and Plymouth via Okehampton could happen without increasing frequencies as the SWR services get moved out of the platform at St. Davids. Adding an extra service beyond that could mean extra works, but it's likely to be limited and could be done within the funding envelope of the cheapest DAL option. Unlike of you needed to do those works to provide extra capacity for new services to run on the DAL.
The WoE line needs improvements by itself. As an ad-hoc user of it IMO most of those right now need to be between Axminster & Exeter ( as planned, to a degree ) but doing some lifting to aid the GWR route to London would be a good idea - that would need a study of customer behaviour ( and future trends in travel ) that's a bit beyond me to decide exactly what to do, but the service is generally liked by it's users as it is. It'd be faster to me to drive the wrong way to Exeter ( let alone the right way to Taunton ) & get a GWR service but I'd rather take the comfort of the WoE, even with the extra hour+. I however am neither a regular or probably a typical customer. People do also want to go north from the SW rather than to London though, so extra ridership does not have to necessarily be absorbed by a different route to the GW routes.
Finding those who would wish to travel from the SWR area to Plymouth is likely to be fairly easy to do, just look at how busy some of the roads heading west are (including those which allows travel from Weymouth to Exeter). Finding who would do it by train is harder.
However having a service running at half hour intervals would make that journey more attractive, as would not needing to change at Exeter with a long official change time.
It is perfectly valid that people wish to head north as well as to London (there's been some busy double Voyagers and XC HST's at times at Exeter which attest to that).
Whilst it's possible for extra capacity to be added to cater for that, again the point that this is potentially 20+ years away comes into play. In that those upgrades would likely to have happened anyway.
Now it might be that there's scope for extra InterCity service between Exeter and Plymouth (maybe a few a day). Without the Okehampton services then that's likely to end up focusing on heading towards Paddington (maybe by going via Bristol so as not to increase paths into Paddington). However if the SWR services are providing some extra capacity then those extra services could focus on heading to Birmingham.
Given a choice of driving, taking the coach, or taking a train to London or points north then the train wins hands-down. Given the choice if I'm going west, then the train is not a clear winner at all and that is from someone who's inclined to take the train over any other mode - it's not a case of not finding a seat, it's a case of it just being slow & not worth the effort. Sadly adding an extra route that is not any faster is not going to change that. I know that goes against what I said about the WoE line just now, but the WoE service east has extra points for comfort ( big selling point for me ) and convenience over GWR ( and of course London has easy local transport ).
The killer time comparison isn't Exeter-Plymouth by road, it's Plymouth-Tiverton Parkway, which is just over an hour in the car ( 10 mins of that is just getting out of Plymouth so if you're on the east side to start with it's even worse ) and an hour and a quarter by train. It only gets worse for the train comparing it west of Plymouth until you're looking at a whole hour longer via train from Penzance.
I can't support a case for a *new* alternate route if it's main point is it's better than bustitution - there's not *that* much disruption at Dawlish and if it's bad enough to affect the railway then it's probably getting a bit dangerous for the inhabitants at Dawlish too, so they need improvements to the seawall regardless if there's a railway on it.
The existing route would be retained anyway, have a via Okehampton route would aid when there's issues, but that's never going to wash when it comes to the business case.
I'd also highlight that the current (fast) services being typically every half an hour also hinders the attractiveness of going by train as that adds to the risk of poor connection times or needing to travel at a time which may not be quite so convenient.
Remove the risk of poor connection times (especially to/from WofE services), remove the need to change to get between Exeter Central/Plymouth and increase the frequency to 3tph or 4tph at a decent journey time (even if via Okehampton was 15 minutes slower) and you'd likely see an uplift in passenger numbers (in fact each of those would likely aid passenger numbers fairly well).
The point about getting a coach (and I agree that disruption is limited, however the building of the DAL would add to that for a time) was due to the fact that often when the route via Okehampton is suggested that those from the existing line say "how does it benefit us, given that we would still need to be bussed" or those who say that "buses will still be fine so we don't need another railway line for times of disruption".
The point I was making was that whilst buses can do the journey quickly they can be slowed down quite a bit by needing to wait for several buses before you can board and that for those who would still need to be bussed there are benefits in not having to bus everyone.
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Could the Okehampton route be made noticeably quicker than the South Devon route if you messed with the alignment? it may be fast to Okehampton but I seem to remember it's very twisty below Lydford.
Given that it's further I doubt it. The best thing for that is the DAL. As I've said before having the route via Okehampton (and the extra passengers that would likely generate between Exeter and Plymouth) would likely increase the business case for it anyway.