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"Fast train" is missing the point; West-Coast Main Line is at capacity south of Birmingham so a new line is needed to relieve congestion and crowding (can't run longer trains because of platform length).
Given we need extra tracks, there is little justification making them slow tracks. You don't save that much by building them to a lower spec.
If you want to save money, tell all the nimbys in Oxfordshire to fuck off and swap the pointless tunnelled sections for cuttings as originally planned.
Pointless fast train, dusty old bombs we'll never use or 5k for everyone in the UK to spank. I know what I'd choose - Paaaaaaaaaaaaarty!