As with previous projects (Channel Tunnel, Crossrail), the Brits are making the mistake of thinking of this project as a tunnel into which a railway will be put, when it should be seen, as the French did with LGV Nord, as a whole new railway which happens to include a tunnel. The distance from Carlisle to Belfast is about 150 miles - 50% longer than HS2 Phase 1. Probably fewer "nimbys", but more difficult terrain to push a high speed link through(A more holistic approach might be to have a three-way junction somewhere near New Cumnock, with a spur to Glasgow, to allow Glasgow-Belfast traffic to benefit from the link as well (and relieve the route over Beattock).