I think that the existing route needs beefing up and protecting, but I don't see the point in a wholesale rebuilding. It doesn't sem to take that long in the scheme of things as a passenger, and if you want a flatter route for freight, the Okehampton line does kust that whilst opening up the rest of Devon.
Exeter to Plymouth is timed at about an hour right now, which is a few minutes slower than a route planner will give you city centre to city centre by road. Given the usual traffic in Exeter that's probably debatable but we're comparing modes in general rather than exact start/finish points.
Okehampton reopening is already opening up the rest of Devon by itself! how many people from Bideford or Bude want to go to Plymouth I wonder, and would they bother getting out of their cars after driving at least half the way. Plymouth is getting more and more depressed as the Navy declines, but it needs to be less the other side of the mountain from Exeter to help that out before being better connected to northwest Devon.
Whilst it would be great to improve journey speeds between Exeter and Plymouth the main reason that is not likely to happen any time soon is that there's no spare capacity for extra services from London or even Reading unless you start using the Bedwyn services and extending them westwards.
However even then it's likely that you'd run into capacity issues at Exeter.
Conversely the SWR services from Waterloo could be increased in frequency with some relatively cheap (at least in comparison to sorting out the GWR route, and probably still coming in at less than the £1.5bn cost) redoubling of the WofE line and maybe a bit of work at Exeter after the opening of Crossrail 2.
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.
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 ).
Now whilst the passengers traveling from London to Plymouth wouldn't typically use the SWR services, with a small decrease in journey time (doing stop/skip on some of the WofE stations) those traveling from SWR stations wouldn't need to go via Reading or Westbury which would aid with capacity on the existing services.
The other thing to note is that actually the Okehampton route would aid the business case for the DAL (Dawlish Avoiding Line) as it would mean that there was more people traveling by rail between Exeter and Plymouth.
Whilst the DAL could provide some extra services along the existing line (although I suspect not) improving services song the WofE line would likely improve passenger numbers more and provide a direct and more frequent connection between Honiton and Plymouth (as well as Exeter) which would aid local journeys.
Whilst it's true that it's possible to bus passengers from West of Exeter at tonnes of disruption, if there was an alternative route (which people would use over a coach, as trains are much more comfortable and the loos are much nicer) then it would mean that it was much easier to get on a coach for those who still needed it.
As an example of you had a train with 300 people on, currently you may need to wait until the 5th or 6th coach until you can leave Exeter. Now if only 1/2 use the alternative route on the train, most people would only need to wait for the 2nd coach (if they hadn't got on the 1st coach) with it being fairly unlucky to have to wait until the 3rd coach.
Increase the numbers on the train and the wait for could be quite a bit longer if you're bussing everyone but even with a train with 600 passengers and half are going by bus then you're still only looking at most 5 or 6 coach loads of people.
However if the numbers being bussed drop to 1/3 of the partners then you're down to 4 bus loads (compared to 11+ bus loads of people).
That's fine if you've got buses going every few minutes, but if you have a 7 minute gap between departures you could end up waiting about an hour if you're on the 9th or later coach. Even on a 10 minutes gap between departures 4 coaches would only be 1/2 an hour
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.
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.
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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.