Surely, given the huge available capacity on the line, the answer to that question is to simply add more trains so that that doesn't happen?
Indeed, the current plan is:
- 3tph to Birmingham (3,300 seats per hour)
- 3tph to Manchester (3,300 seats per hour)
With a through station at Birmingham that could be
- 6tph to Birmingham and on to Manchester (6,600 seats per hour)
Benefits; HS2 has never been able to properly claim the benefits of released capacity because those items were beyond it's remit. Building a local transit system/regional rail should have been part of the projects purview even if the actual building and possibly funding of that may have been done by regional authorities.
Whilst that true or could have given broad brush examples:
One platform used by 2 long distance services an hour could then be used by 3 (probably 6 as they're half length) local trains an hour, cut those services are from other platforms, which can now run longer trains.
Because the same cost drivers that have driven the cost of HS2 into the stratosphere will have done it for a 125mph railway.
Meanwhile you get a railway that is of no use at all for relieving the MML or ECML, and thus will never be filled.
The near doubling of Euston isn't down to speed of the track.
The higher speed means lower rolling stock (and staffing) costs, as you need fewer trains (and therefore fewer drivers and guards) for the same service. HS2 will have longer trains to Manchester, at the same 3tph frequency, requiring fewer carriages
For example London Manchester:
Current trains (each hour 11 coaches, 11 coaches and 9 coaches) 155 coaches for the 5 hour round trip
Alternative route with a mid way speed, which cuts journey time by 30 minutes each way but with 16 coach (400m) trains 192 coaches for the 4 hour round trip.
HS2, cuts journey time by 1 hour each way, with 16 coaches 144 coaches for 3 hour round trip
On the old rule of thumb of £100k per coach per year that's an extra £1.68bn over 35 years of the trains life (plus the extra staff costs). That rule of thumb is probably old enough that least costs would be double that now, so between £3bn and £4bn extra.
Whilst not 9% of 100bn, it's made a fair debt in that difference, the extra speed would likely also make it more likely that people would use it, which would mean more income.