One of the most costly elements of the proposed high speed railway, and one that will take the longest time to construct, is the subterranean terminus at Euston and the 10km tunnel to the Crossrail interchange at Old Oak Common.
The long tunnels will mean that the trains will have to be built to the special stringent standards of fire resistance required for underground trains, adding considerably to the cost.
If an alternative surface route could be found, this requirement could be relaxed, with consequent savings.
The previous posting suggests that there is spare capacity at Euston, currently taken up by the lightly used London suburban services operated on lines with DC electrification. This may be the key to avoiding the great expense of a new terminal and all the tunneling.