Fantastic - that's my plan - that rather than plugging it to HS2, you use the existing Airport line to go into Manchester!
Oh wait, you seem to not know that
Manchester Interchange is the name for the HS2 station near the airport to distinguish it from the existing Manchester Airport station.
The HS2 website has not yet filled me in on this latest idiotic PR exercise.
I thought Manchester Interchange was the new PR name for NPR's Picadilly low level.
What trains aren't stopping? There's no capacity gains to eke out by doing that as it's already been done. The problem is perhaps more all the merging and diverging - by adding in Cheshire and Wales services you've effectively made sure that similar problems to the Castlefield corridor of lots of diverging and converging with a two-track bottleneck are created. Only this is a 140mph+ section of intercity track, not a 30mph city centre crawl.
Goodness me, this is the source of all the misunderstanding isn't it? That you think that Piccadilly would be renamed 'Interchange' and so every time I said 'Manchester Interchange' you read 'Manchester Picadilly' instead of 'Manchester Airport HS2'.
The Mid Cheshire line crosses the proposed HS2 route about 3,000m south of Manchester Airport (HS2) station, which is the beginning of the 230km/h speed limit.
Castlefield corridor's problems are not really caused by diverging and two track bottlenecks, but by multiple flat junctions that are positioned close to one another, which is not a problem here.
Trains that are stopping at the Airport(HS2) will already be moving quite slowly, and trains that are not will still be moving slower than might be anticipated because they will be braking for the speed restriction north of the airport.
I dont think there are drastic losses of capacity from 1tph/2tph turning off onto the Mid Cheshire line.
And even if it became a problem I doubt four track would be needed rather than a single line running from the southern approach of the airport station.
I don't have an IEP acceleration curve, but a Class 395 (demonstrating what a modern EMU can do) would be able to accelerate to 100mph and then brake down to 60mph again in the 3,000m space.
With a complete running time of about 100 seconds.
Slower than a continuously accelerating unit, but even at 230km/h continuous we would be looking at a running time 47 seconds.
We lose something like a minute, which is much less than the design headway.
If we stopped all trains at Manchester Airport(HS2) station this change would be even smaller.
EDIT:
Attempting to use the existing line to the airport would require even more track and somehow picking a way through the airport infrastructure, and then you end up on the crowded side of the Picadilly throat (ie. the Castlefield Corridor/Styal line side).
Even if you require 3,000m of track from the divergence point to the Manchester Airport-HS2 station, you are not going to get a connection to the Airport line that is as useful for that much money.