I understand that fixing the junction at Colwich is constrained by available space.
There is a lot of line between Colwich and Rugeley, surrounded by farmland. Is it not possible to build a junction further back, where there is more room and run two parallel two-track railways between there and Colwich?
It would be very easy (but not cheap) to add a viaduct somewhere between Rugeley and Colwich to grade separate those two flows, but that won't fix all the constraints around Colwich. Freights coming south from Crewe on the Up Slow line have to cross over the paths of northbound down fasts and southbound up fasts to get to the Up slow at Rugeley which is on the east side of the WCML alignment. And these all have to fit through the twin-track Shugborough Tunnel.
And running all Crewe traffic on just two tracks from Shugborough to Rugeley extends the length of the Shugborough bottleneck to several miles. Full grade separation of the Colwich / Shugborough / Stafford Trent Valley complex would be extremely expensive.
Hixon would seem to be a strange end location, only being able to serve trains to Manchester.
Manchester is by far the most important destination for trains on the northern WCML.
Taking HS2 to Hixon buys a surprising amount of extra capacity, for just 10 miles of new high speed line. Firstly, by eliminating the fast paths crossing at Colwich, it simplifies operations in the Colwich / Shugborough area so should increase the overall capacity of that bottleneck.
And secondly, it means that HS2 trains from London can use both the 2tph Avanti paths from Euston and the 2tph Cross Country paths from Birmingham to access Piccadilly. On the existing rail infrastructure in South Manchester, HS2 could run
- 2tph OOC-Piccadilly direct
- 2tph OOC-Curzon St(reverse)-Piccadilly
- 1tph OOC-Macclesfield terminator
That's 4/5 x 550 seats per hour into Manchester, plus whatever goes via Crewe. Extending HS2 phase 2a to Crewe means 35 miles of new line, compared to 10 miles to Hixon, and would be more than three times as expensive, given all the grade separation viaducts needed for HS2 south of Crewe. And would give no more paths through the South Manchester suburbs.
Note they would need to extend the Stoke stopper to Wolverhamption to serve Stafford-Stone-Stoke traffic.