This is where today's railways makes no sense and totally infuriates me! Getting more out of the Voyagers now by running extra paths to Exeter will almost certainly mean capacity being lost elsewhere. In the meantime, we have got sidings starting to fill up with redundant HST's. I know people will scream "they are not compliant, toilet tanks, doors etc." And the fact XC pays more access charges to run a HST, but there it is, ludicrous restrictions to an obvious solution, the HST is a great, no nonsense, mass people mover.
More XC HST's on Plymouth/Newcastle/Edinburgh corridor would free up more Voyagers to "double up" plus offer better resilience in the Dawlish area in times of bad weather. Total sense.