To get this forum back on track here's some food for thought. Currently trains on this route stop at every station and also wait in loops for paths. If the line was double track then the journey timefor a non-stop service between Exeter and yeovil would be around 40 minutes (personal experience of non stop run behind a class 50 in 2008 and a couple evening HST diversions) and Yeovil to Salisbury would be around 35 minutes (Already timetabledwith the 00:09 service from Yeovil Junction to Salisbury). Thats a potential journey time from Exeter to Salisbury of around 1 hour 15 minutes!!! A saving of 30 minutes. Coupled with todays 1 hour 20 minute timing from Salisbury to Waterloo thats a total journey time of around 2 hours 35 minutes. An increase to 100mph from the current 85mph would bring that down even more.
To put it in perspective the average journey time from Exeter to Paddington is around 2 hours 20 minutes.
Apologies for restarting this thread late in the day, but an interesting topic and bit of time on my hands after Xmas, so here goes!
Essentially what you're proposing with your 2 hour 35 minute timings is a reintroduction of the pre-September 1964 Southern Region express service - fine for Salisbury and Exeter but not much good for Sherborne and Honiton. In 1963, the Southern and Western Regions did a survey of the competing routes to the South West of England and found that five times as many West Country passengers used the Paddington route to Exeter and beyond, and that, although the SR route was shorter, the WR route was quicker. They also concluded that concentrating expresses on the SR route would either require a new spur at Exeter, or time wasting reversal of all Plymouth and Exeter trains at Exeter St Davids (no HSTs or DVTs in 1963!). Focusing expresses on the Southern Route would also have resulted in a less attractive direct service from Taunton to London.
Given the arguments above and also that the Southern lines west of Salisbury were losing money heavily from the late 1950s (in 1957, these lines were failing to cover direct costs by £800,000), the Western Region announced in February 1964 that the WR route would be the trunk route with the SR route restricted to serving a limited number of intermediate stations between Salisbury and Exeter. Over the next 3 years, the WR removed loss making branch lines (e.g. Axminster, Seaton, Sidmouth) and Salisbury - Exeter stopping services, and further cut costs in 1967 by singling the line, leaving the 2 hourly semi-fast service which ran in various iterations until December 2009.
You can argue in retrospect that the WR pruned the services too much, but the cost cutting made sense at the time, and their plan to focus on semi-fast services was, in my opinion, the right one for the long term survival of the line.
Now you could argue that Chiltern Mainline provides an example of redoubling and faster services which could be used as a template for reintroducing Waterloo - Exeter expresses. However, Birmingham is a much larger city than Exeter and the amount of redoubling required (and cost) on the Salisbury - Exeter route is much more than Marylebone - Moor Street. This leads me to conclude that redoubling throughout and reintroducing a duplicate express route to Exeter would not be a cost effective use of public funds.
However, I do agree that Waterloo - Exeter services should be accelerated with the introduction of more and longer passing loops e.g. at Tisbury and Whimple and the removal of Feniton, Whimple, Cranbrook (in 2012) and Pinhoe stops. These stations would be served by a new local service from Honiton to Exeter. With relatively limited investment in infrastructure, these potential developments should allow a reduction in Waterloo - Exeter times to 3 hours.
