The capacity constraint arguments revolve around the BCR, which the report gives as follows:
|BCR without WEI
|BCR with WEI
On the basis of these numbers, the select committee conclude that HS2 represents "high" value for money because HMT classifies BCRs above 1.5 as good, above 2.0 as high.
But this is a little misleading, because the BCR for transport projects tends to be higher than for other investments (part of the reason why some people argue that we tend to underinvest in transport in the UK). Here's the picture for a bunch of DfT projects taken from Eddington's 2006 report (figure 3.1):
These figures include some wider benefits (Eddington doesn't have a figure for the traditional CBA) so you need to compare to the second row of the transport select committee table. To read these 'box and whisker' plots notice that the box captures the 25%-75% range of project BCRs with the big horizontal line capturing the average. On my reading, a wider BCR for HS2 of 2.0-2.6 puts it, at best, in the bottom 10-15% of projects that DfT had on its books at the time of the Eddington report. I am not sure that these numbers are 100% comparable, but I think that they are roughly right. If so, that suggests that the case for HS2 remains weak when compared to many other transport projects.