If a vocal supporter of HS2 says they're in favour, that's to be expected.
If a vocal critic says they're against, that's to be expected.
I'm interested in his contribution because it underlines very valid concerns that the budget is out of control. I've asked on this forum numerous times if there is an upper limit to the budget amongst the supporters and in some cases there appears not to be. Personally that is a concern of mine.
An increase of costs isn't linear compared to the benefits.
As the infrastructure costs increase the income from the trains stay the same, in doing so they are linear.
However the extra costs of running trains for 1 extra person is virtually zero, however the extra income is almost all of the extra ticket cost.
Therefore if you double the costs you don't need to double the number of extra passengers to still result in the same total benefits when related to the costs.
It's why I keep coming back to the point that rail passengers are above the level which they should be to be on track to meet the required passenger numbers to justify the costs.
Now on the original requirements were for circa 100 million passengers a year.
Starting with the base level of 31 million passengers between London and the regions which benefit from HS2 in 2009 and assuming that 1/3 of those passengers and that 40% of HS2 passengers start/end in London, that gives us a baseline of 20 million passengers from London and 50 million overall. (This corolates with the expected doubling of passenger numbers).
Now by 2018 the growth model had assumed that (using 2.5% growth each year) that there would have been 24.9% growth. As can be seen from the table below that this growth had been 49% across all regions.
View media item 3340
That means that if we saw 2.5% growth from now until 2037 (bearing in mind that Virgin had recently seen 3.5% growth, so isn't unreasonable) then rather than having 100 million passengers we'd see 120 million passengers.
To carry that many passengers wouldn't need any extra infrastructure or any extra staff, however there's 20 million extra passengers each year. If we assume that the average ticket is £40 for each of those passengers that's £800 million. Even at an extra £50bn that's enough income to cover interest payments of 1.6%, however the UK bond rate is half that at just under 0.75%.