I can do no better than link to a thread on exactly the same subject that I started two years ago.
If you were planning HS2 . . .
My answer would still be based loosely on the Great Central, absorbing part of its trackbed where possible, but used mostly for the slow lines (125 mph) rather than the fasts (200 mph). The route would split at Sheffield for links to Leeds-York-Newcastle-Edinburgh and Manchester-Preston-Carlisle-Glasgow, using existing lines until the new ones were ready. The idea is to absorb Manchester and Leeds traffic, allowing Virgin to divert some Manchester trains to Birmingham - similarly the Leeds express on the ECML could divert to Hull or Cleethorpes. However, and far more importantly, it would essentially take over from a 110 mph diesel line, cutting at least 30 minutes off the time to Nottingham and 45 minutes off the time to Sheffield, plus giving Bradford and "Cleckhuddersfax" a fast, direct service to London. XC would be able to gain several new routes, potentially avoiding Birmingham if they want to. The intention though is
not to take over from the two current Anglo-Scottish lines, but to act as a temporary relief line down the middle for about ten-to-twenty years, by which time HS4 (London-Birmingham-Manchester/Liverpool) and HS5 (London-Stansted-Leeds/York) would take over. The ECML and WCML would
not be cut in two, with their southern halves isolated from the northern halves. However, the MML might lose its Sheffield and Nottingham expresses, to be replaced by local and freight traffic. BTW, HS3 goes to Cardiff and Plymouth.
