To my mind the logical thing to do would be to run an hourly service calling at only Reading, Taunton, Tiverton Parkway and Exeter (and beyond)
I would also call all the fasts at Westbury as its such an important interchange, especially with the improvements on the Westbury - Swindon line and this would provide an hourly single change journey from Plymouth and Cornwall to Bath and the Solent area. The Westbury avoiding line could be abandoned, the junctions removed (saving maintenance/renewals) and the alignment straightened for running faster into the station, then the former trackbed could be used as an alternative route for a town road bypass, also potentially giving much better road access to the station area and the nearby industrial areas. The station could be improved as a major parkway, with its fourth platform reinstated (as, I believe, already suggested/proposed).
and a new hourly semi fast service calling at the following:
Reading
Newbury
Kintbury (limited service only)
Hungerford
Bedwyn
Pewsey
Devizes (new station on A342)
Westbury
Frome
Castle Cary
Somerton (New)
Langport (New)
Taunton
Norton Fitzwarren for WSR & Taunton P&R (New)
Wellington (New)
Tiverton Parkway
Cullompton (New)
Exeter St Davids
Exeter Central
Nice but you missed Bruton!
I would keep the Bedwyns separate doing:
Paddington
Reading
Theale
Thatcham
Newbury
Kintbury
Hungerford
Bedwyn
electrified and extended to Marlborough as has been suggested.
Then the next tier could go:
Paddington
Reading
Newbury
Hungerford
Pewsey
Devizes Parkway (new station)
Westbury
From Westbury there are many alternatives:
Frome
Radstock
OR
Frome
Bruton
Castle Cary
THEN
Somerton (New)
Langport (New)
Taunton
OR (alternate hours?)
Yeovil Pen Mill
Yeovil Junction
(filling in gaps in Bristol - Weymouth service)
END
Any new Exeter - Taunton stopping service I would probably send to Bristol via Weston Super Mare in preference to London via Westbury as that would probably have more local traffic potential, and the fast trains at Tiverton Parkway and Taunton would be more attractive for the London bound traveller (who could also interchange from local stops into those expresses).
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Regular hourly interval Reading, Taunton, Exeter, Newton Abbot service pattern would also help with fitting in the much needed half-hourly Devon Metro.
Yes agreed, the NR Route Study suggests regular clockface paths for the expresses between Newton Abbot and Exeter could help considerably in that respect, but with the B&H service as it is today that is impossible.