But the tunnel from Manchester Airport to Piccadilly will be full, because of the headway constraints for emergency escape.
The tunnel on approach to Manchester is 12.67km long in the last set of planning documents.
There are four intervention shafts - at 2.6, 4.7, 6.8 and 9.2km from the southern end.
The absolute longest distance between access points is 3.5km at the Manchester Picadilly end.
The stated HS2 journey time from Manchester Picadilly to Manchester Airport is around 7 minutes, for a distance of around 16-17km stop to stop, for an average speed of 137+km/h.
Even at that average speed, which includes the station throats at both ends, you'd traverse the 3.5km gap in around 90 seconds. I'd suggest given that the train will have been rolling for a kilometre before it gets into the tunnel that the actual time will be significantly less, given how well modern high speed trains can accelerate.
Given that the headway for the nominal 18tph capacity of the core is 200 seconds, I am skeptical that this will form a real constraint on capacity.
And even if it did - by far the cheapest solution would be to add an extra intervention shaft to the plan, even if it costs a billion pounds it will avoid 15 or 20bn in extra high speed line costs on the duplicate railway to Leeds
Nor will Piccadilly HS station have capacity for reversing more Leeds services.
Why not?
It's got six platforms.
Numerous termini handle the required number of trains per platform, and we have the advantage of a comparatively simple route knowledge requirement for LU style reversing, and thats before we consider more exotic options like autoreverse.
Fenchurch Street is instructive
If the 3tph Euston - Manchester services were extended to Leeds, they would likely become overcrowded.
Even if we were limited to 3tph (and I don't think we would be), that's at least 3300 seats per hour. And given we would have continuous GC track to Leeds, if necessary we can push to 4500 (TGV M).
Even 3300 seats per hour is a huge uplift over the current service, it will be a looong time before that is exhausted.
EDIT:
Using the old stats from the
Sapsan, the train will be going something like 100km/h before it even enters the tunnel, and will keep accelerating for those 3.5km to the first intervention shaft. Given that the ruling linespeed of that section is 250km/h.
EDIT #2: At least one reading of this document implies that this restriction is not limiting on HS2 because it has adopted the "alternative technical arrangement" using cross passages between the running bores.
I think I got confused by that.