Time and time again it is said on here that any electrification west of Bristol/Newbury should end at Exeter. What is the point of this unless you are going to start terminating the majority of intercity services at Exeter? Currently most intercity services continue to Plymouth so if you don't want to electrify to Plymouth (which it seems there is quite a bit of opposition to on here) then you may as well end the primary intercity network at Exeter and run a DMU service from Exeter to Plymouth. This would save needing bi-mode trains for just the last 50 miles of a long journey from London. For XC (assuming all the gaps east of Exeter are filled in) it's even worse as you end up with a situation where a train from Edinburgh runs under electric power for hundreds of miles all the way Exeter but has to cart around diesel engines just for the last 50 miles. Much better to just terminate the service at Exeter if the wires are going to end there. If there is still demand for through services to Penzance then these could be run by 222s as there would at least be 130 miles of running off the wires from Exeter to Penzance as opposed to just 50 to Plymouth.
That raises a very interesting point about the lines west of Exeter. The scenic coastal stretch past Dawlish has been a headache ever since it was opened and it isn't getting easier with the passing of the years. Stand at Dawlish station when the wind is in the wrong direction and you will get soaked on the up side platform as the waves break on the sea wall and send a shower of spray all the way across. The ballast has had to be glued down. I wonder even if it would be practicable to electrify the route.
There have been proposals to construct a direct route between Exeter and Newton Abbot, I understand it has been looked into but would involve the construction of a very long tunnel. No doubt there is someone here can shed more light on that.
Beyond Newton Abbot the route is steep and winding. This is tllting train territory if you want to run fast. The GWR considered electrification in the 1930s. To some extent between Reading and Taunton is also tilting train territory.
If there was not such resistance to locomotive haulage, Exeter would be a perfectly good place to do a changeover. Electric push from London, diesel push from the west. The London end driving trailer could have a blunt end as there is no significant high speed running west of Exeter (and via Westbury, not that much east of it, either.)
But IEP solves everything, we are assured.