If line speed improvements were to be made somewhere I'd be looking to try and improve the Reading-Taunton line, in a bid to improve journey times to Devon and Cornwall, rather than go for 140mph running on the core GWML. I wouldn't be surprised if 125mph wasn't possible in HST running because the only sections straight enough, all else being sufficient, were too short to both accelerate to that speed and sustain it and/or some congestion exists on the easternmost part of the route. If an 802 can get to 125mph faster on diesel than a HST would, the 110mph stretches could be upgraded to that, and the 100mph stretches up to 110mph. At the moment Paddington-Exeter is just over 2 hours, to Plymouth just over 3 hours, and to Penzance around 5 hours. If it was possible to enhance line speeds and improve capacity in places to reduce journey times by 10-20 minutes on current workings, then railway journeys can be sold on a "less than 2/3/5 hours" to those major destinations. In an ideal world rerouting the line through the Bedwyn area to be straighter and faster would help, but I imagine be too costly and subject to local opposition.
