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//the existing Rail Network should be upgraded instead.//
not an option, unless the travelling public will tolerate 10-15 years of continuous disruption - the West Coast upgrade was started in 1990s and is still not finished - indeed there was major inconvenience to passengers at Birmingham, Liverpool and the north-west this weekend just gone, for the same reason.
if rail freight were to disappear, the need for high speed rail would disappear with it, overnight. this will occur when 60t trucks are allowed on the roads, as the fiscal case for rail freight will vanish forever, and will provide all the extra capacity the rail network needs. but, this option will require major road upgrades to accommodate.