It's really not the defining limitation.I'm amazed that the single line between Cowley and Crediton has that many spare paths on it ? 8 Trains PD Okey, How many to Barn? Plus leaving space for, say 1 up and down ballast per day at some point. - Are they going to combo and split at Crediton?
Are these 159 SWR extensions or "new" use of GWR 150's ( or what ever?)
Cowley Bridge Jn-Crediton is 5¼ miles of single line, 5 miles of that at 60/70mph. Even with a stop at Newton St Cyres, the Sprinter SRT is 7.5 minutes (9.5 in the up direction including a minute's engineering allowance).
Add on the standard 2 minutes for resetting the route after the passage of each train and you have a line occupation of 21 mins out of each hour for your standard hourly Barnstaple path.
As you can see, in theory you could operate 5 trains an hour over that stretch, so having an hourly Barny train in each direction plus a two hourly Okehampton service is not exactly taxing.
Most services don't call at Newton St Cyres either, which saves a further 2-3 mins.
These are GWR services, using Sprinter (150) timings.