Different stock, I think - Barnstaples are 158s at the moment, aren't they? it's a fairly long route with few stops, vs Exmouth-Paignton being the other way. Running the Honiton/Axminsters out to Okehampton might work, if there's any point ( from a utilisation point of view rather than people wanting to go from Honiton to Okehampton a lot... ).
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NR's document on the WoE line accepted it probably needs work, no plans for more than adding some new loops (
I think this is the right NR document )- definitely not redoubling the lot at the moment. Needs more flexibility at the Exeter end ( but that's been done to death recently in other threads ). From browsing 50s timetables the current service is still quicker to Exeter bar the ACE, which was 10 mins faster... ( tube to Paddington & GWR still thrashes both, obviously ).
If you want people to use trains to take them to the seaside, best ask for trains that can take them to the seaside, not to Plymouth!