The HS2 rail link announced yesterday which is likely to be built not by 2026 but frankly much later. Without Government pump priming and tax support for construction companies it is virtually a non-starter.
The original idea by Ove Arup was to create a hub at Heathrow and then link into Crosssrail which is due to be completed in 2015-ish.
If the notion of HS2 is to help the economies of the major Midland and Northern cities like Birmingham, Leeds and Manchester then build those links first. Otherwise they'll never be started let alone finished.
The idea of rebuilding Euston is frankly a non starter anyway. The hub at Heathrow was much more credible and much cheaper.