I've heard that as the main reason for lack of electrification of the WR as a whole - and it makes sense in many ways because otherwise you'd have to:
- Electrify secondary routes such as the Cheltenham branch from Swindon or the Worcester route, which could be inefficient use of funds;
- Electrify only the core routes (Bristol both ways, Swansea, Oxford) and either run diesels under the wires or run diesel shuttles for the secondary routes.
So it's somewhat problematic either way, although one could have envisaged a pattern in more recent times in which the Bristol and South Wales routes were electrified, and the secondary routes as well as the west of England line were HST-operated. That way you'd have a mix of electrics and HSTs running all the services which wouldn't have had major pathing problems, presumably, as they would be similar speed.
Before the advent of the bi-modes it must have been quite a challenge diagramming the fast lines out of Paddington, as you had to fit one to three Turbos an hour (Bedwyn always, and some Oxford/Worcesters) in between HSTs and 180s with higher max speed. I recall a major timetable recast around 2004 which provided a more regular departure pattern and they did achieve this - I think one key factor was that the Bedwyn (xx18; Turbo) was non-stop to Reading while the following Oxford or Worcester (xx22) called at Slough which would presumably allow an HST or 180 to be used without catching up the preceding Turbo.
