I cannot find the reference just now but I seem to recall that proposals for upgrading the GW main line include provision for 140 mph running. This strikes me as strange. Although the GW main line was laid out for high speed, such has been the development of the London-Bristol corridor in the past 30 years that the character of the route has completely changed. There is little opportunity or need for high speed running because the trains have to keep stopping every 15 or 20 miles to pick up or set down passengers. The trains will hardly have had a chance to wind up to 140 mph before the brakes have to go on for the next stop. Slough, Reading, Didcot, Swindon, Chippenham and Bath are all busy and popular local transport hubs serving areas which have become suburbanised, to the point that then entire belt between London and Bristol is turning into an arm of the south-east conurbation.
Bearing in mind that costs are proportional to more than the square of the operating speed of a railway, somebody ought to be asking whether the normal top speed on the route should be more than 100 mph.