Sorry, I did a bit of a brain dump and so it didn't flow very well, I'll try again in shorter sections below:
I'm just guessing here, but is it possible that the calculation is that spending £500 million today (if that's the correct figure) is cheaper in the long run than having to close the line and clean up the mess every time there's a rock fall or an exceptionally strong wind, and therefore represents the best way to keep the line open (given that closing that line would be politically unacceptable even if an alternative route was built)?
To get back £500 million project cost over the lifetime of the project (for example over a 50 year period) either each rock fall would need to be very costly and/or happen fairly frequently.
For instance it would require 5 rock falls each year over that 50 year period, with each one costing £2 million to clean up/in lost revenue to just get back the money spent.
It should be noted that although there would be savings in maintenance of the rock fall prevention systems which are already in place. This means that the above calculation is fairly simplistic.
However, even allowing for that benefit which will improve the business case you would still need a fair amount of maintenance savings to get to, say, a figure of 1:1.2 or higher.
Given that the out to sea option is only proposed to fix part off the problem (that being the rock fall issue), it will mean that that there's still going problems that could lead to the old route being blocked when either of the other "traditional" options wouldn't be blocked.
This compares with the other options which are summarised below.
The Dawlish avoiding line (DAL) would cost about 3 times as the sea option but in addition to solving the rock fall problems would :
- increase capacity (in that the stoppers could still use the coast route, which would then be easier to keep operational with fewer trains on).
- reduces journey times (by about 10-15 minutes quicker).
There could even be scope for an inland station on the DAL. Although this station may not have many services during normal operations it could be a useful point to run replacement bus services from.
This replacment bus service could be to some or to all of the places along the existing line from this station (either as a hub or as a point along the replacement bus service route so people are on buses for less time), so as to reduce the delay associated with running buses when the line is closed.
The route via Okehampton would cost about twice as much as the sea route. However it would make getting around by public transport in this area a lot better.
Also when diversions were required for those heading to Plymouth/Cornwall (with the train running via Okehampton) the added delay would be fairly small.
Although those heading to/from South Devon would still be reliant on bus replacement, however rail replacement buses would only be serving South Devon and so you would more sure that you were going where you wanted.
In addition when getting on/off the trains at Exeter it would be a lot easier as only some of the passengers would be doing so.
As such, although it's not ideal, very few places can avoid bus replacement services totally, even some fairly major places like Milton Keynes.
For instance even with the DAL option if there works at (say) Totnes there would still be a need to get everyone on to replacement buses. With this probably being done at Exeter due to the nature of a lot of the existing stations (although again a station on the DAL could be useful in that regard).
The via Okehampton route already has the best (published) business case so unless this new option has a better business case then many would still be looking at having one of the two favoured options go forward.
If there was a way of improving the business case for either option then these should be looked at. I think the DAL the could include the following options to improve the business cases:
- a new station to generate extra passengers
- running more trains (maybe, as an example, even looking at what benefits you gain by reinstating Waterloo - Paignton services)
- a way of reducing costs
- combining it with another project (say electrification) to see if the combination gives a better answer.
Likewise with the via Okehampton route, these could include:
- running through services between Plymouth and Waterloo, on the Western end of the WofE line (where there's an aspiration for more services)
- running through services on XC and/or GWR. Possibly running one or two services a day, mostly for staff training. Such a service, if there was demand could possibly call at Okehampton and/Tavistock, giving direct connections to Bristol, Birmingham and beyond or Reading and London.
- a way of reducing costs
- combining it with another project (say electrification or some redoubling of the WofE line to provide for 2tph on the Western end) to see if the combination gives a better answer.
Just because the first answer isn't quite as good as it should be to normal get funding shouldn't stop people from making suggestions which could result in the answer being better and therefore more likely to be built.
As I understand it the main problem with running through services on the new line would be the risk of a problem on the new line causing delays elsewhere.
Although this is a real concern I do wonder how much of an impact it would really be and why this isn't cited as a good reason to split services (especially those like XC services which can get delays from all over the place).
For instance if someone suggested that the WofE line services split at Yeovil so that the mostly single track line doesn't cause problems into Waterloo, they would be shoot down in no uncertain terms.
Yet proposing that changing from the existing situation where people change at Exeter for Plymouth from the WofE line is (epically bearing in mind that the route via Okehampton would involve a lot of double track with a lot of measures to try and ensure it's reliability and would be about 80 minutes of a circa 5 hour journey) is treated by some as a crazy idea and why would we do this.
Even though such a through service would make rail travel from a large area of Southern England (either direct or with one change) to Okehampton, Tavistock and Plymouth a LOT more attractive than the existing options.
I wonder if a service between Portsmouth and Cardiff didn't exist how much support that would receive of it were suggested, yet now that service is in need of longer trains.
As suggested above, if through services to Waterloo are too risky (or even in addition to them), there could be scope for XC services from Manchester or Leeds to be routed on the via Okehampton route.
If this was paired with a unit (joining/splitting at Exeter) that also run to Paignton then XC wouldn't miss out on much income (or could even have more) but would have a longer train running through the XC core (i.e. 8-10 coaches to Exeter and then 4-5 coaches between Exeter and Plymouth/Paignton).
It would also mean that the via Okehampton route could directly benefit South Devon in that they would have more XC services.
Such a XC service, with a station stop at Okehampton or Tavistock, could be very beneficial to the local tourist industry as it would be very much more on people's radar (in that it would be seen on timetables, there would be station announcements and prior would go through it on the train).
It's likely that with a circa 65 minutes journey time between Exeter and Plymouth it wouldn't be that much longer than the other unit running the service to Paignton. This would mean there shouldn't be too much of a miss match of journey times between the two units (if needed the Paignton unit could run an extra shuttle service up to Totnes and back to get a closer connection time) for connecting the two units back together for a return journey from Exeter heading north.