This is a really interesting question. Hasn't the passenger growth on the Southern WCML over the last decade exceeded the 2.5% assumed in the HS2 business case?
If so and the growth continues at the current rate I can see any decision to cancel phase 1 of HS2 coming back to haunt the government later this decade as as the lack of capacity becomes more accute over the coming years. I carn't see what upgrades can be made on an already congested railway where trains are already 11 or 12 coaches long.
All those that are opposed to HS2 and say we should upgrade existing lines instead never actually specify what works that would entail, how much it would cost or how long it would take and what would be the disruption to existing services in the meantime.
2.5% growth year on year would get us to 25% growth over the 9 years from 2009 to 2018.
As you'll see from the table below this is an average of 49%:
View media item 3340
However you'll also notice that for the WCML this is mostly 70%.
Percentages sometimes are hard to understand or can be miss quoted, if we look at in terms of seats.
If we assume that from a coach of 70 seats that there were 30 in use in 2009, by now that would have gone up to 38, the average is actually 45 and for the WCML it's 51.
As such there's not a whole lot of spare capacity left in that coach, whilst it's true that there's been capacity improvements to provide extra coaches it should be remembered that 2.5% of 170 is more than 2.5% of 100 that coach is going to get very busy very quickly, even with those capacity increases.
Now to deal with the "we'll just a few more coaches", sorry but it's not going to work. The 11 coach 390's have 589 seats whilst the 9 coach ones have 469, that's a total of 1,048 seats.
A HS2 train is likely to have 1,076 seats (assuming same density as an 80x), as such HS2 will be providing a 2 to 1 capacity increase over the current trains.
That's a lot of extra capacity, nothing anyone has suggested is likely to get close to that.