Some good points made, Grecian (and a good win yesterday
).
Had to drive down to Truro a couple of times last week, joining the A30 at Whiddon Down. If, and I mean IF, a new higher speed line were to be down to Cornwall, then surely this would be the route. Unlike the south coast of Devon and Cornwall, it's rural all the way, with long shallow gradients and no need to knock down numerous properties.
Furthermore, there is the potential for a junction at Okehampton/Meldon, the A30 bisects the Newquay line (Park and Ride/junction, and could easily rejoin the main line into Truro. Bodmin Moor would rightly be an issue, but it's not actually that big, unlike Dartmoor, and could be circumvented or even tunnelled under.
So, Paddington to Exeter is fast, even quicker if electrified - just need to fix Cowley. Reverse at St Davids (two minutes for a modern train), and then forwards on the upgraded, redoubled Okehampton line. Nice former main line with potential for 100mph running, if the two level crossings are removed (only serve minor roads). Also plenty of space on the level between Crediton and Newton St C. to install running loops for the local services.
Join the new line either before Okehampton, or between there and Meldon, where we meet the A30 - and avoid Meldon viaduct. Bang - 125mph all the way to Newquay Parkway/Junction and Truro alongside the A30/A39, with Truro the new hub for local services to St Austell, Falmouth, St Ives etc. This gives a line speed of 100 - 125 mph from London to Truro, knocking off, what, ninety minutes? Two hours for a non-stop service?
The issue would remain as to how best serve Plymouth (I'm assuming the former mainline would remain open for local services, freight, sleeper, diversions etc and can't see why not). Plymouth to Okehampton via Tavistock is of course former main line, could be easily redoubled, and IF it was electrified for the gradient, easily 100mph running. Bypassing Tavistock would be costly and controversial, however. But with the necessary funds, it could be done. Otherwise, electrify the existing mainline and build that tunnel through the Haldon Hills, then couple to the Cornwall train at Exeter (cost savings), and onwards to London.
And then I woke up...