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Ron.....I am no railway expert, and I expect there is someone else on here that can give you a fuller answer, but ...........
The tracks already in place just cannot provide a safe and fast journey, for these new high speed trains. They are too twisty, for one. BR did try to run high speed trains in the past, the so-called Tilting Train or Advanced Passenger Train but it was a technical failure....nobody wanted their tea or coffee swaying about at 152 mph, and the plans were abandoned. You an still see the APT in York.
So a completely new track is required, at huge cost.
I have only travelled on the HST on the Eurostar and it is a very impressive journey. very smooth and very quick.
When I first lived in Swansea, in the late 60's, it used to take over 4 hours to get to Paddington, on the old trains. The same journey now takes less than 3 hours, in the Inter-City 125, a great improvement ! Its a great train as well...smooth and not bumpy at all.