And we are yet to discover what kind of fare increases will be necessary to pay for HS2 construction & operations
There may not be fare increases, yes HS2 will cost a lot to build, however in the last decade there's been £30bn of investment in enhancements to NR infrastructure.
However even if that wasn't the case, the cost of running HS2 services will be cheaper.
Take for example the number of coaches required to run a mix of 9 and 11 coach 390's at 3tph taking 5 hours to undertake the round trip between London and Manchester that's between 135 (all 9 coaches) and 165 (all 11 coaches) coaches. How many more do we need to run HS2 services with 16 coaches?
Actually broadly the same, as the round trip journey time reduces to 3 hours. It works out at 142 coaches. However the capacity had significantly increased. Rather than the total of seats from a 9 coach and an 11 coach train coming in at 1,048 seats a single HS2 service is expected to have 1,100 seats.
However that's only part of the benefit of the faster journeys, now currently (assuming a driver and guard stay with the same train all shift) over 8 hours you could run London, Manchester, London, Manchester then stop. With HS2 there'd be enough shift to do London, Manchester, London, Manchester, London, Manchester. As such to run the same frequency of service you'd need fewer staff.
(Clearly such a pattern would never work as you'd be ending shifts away from where you start, however it's useful to illustrate the extra that the same staff could do).
Now whilst energy costs will increase, they tend to be a fairly small cost, with things like lease costs and staff costs being fairly significant costs, all of which could be reduced.
If the costs per seat are lower then the income per seat can be reduced. For example if costs stayed the same (lower staff costs, lower least costs, higher energy costs, higher track access charges, etc.) and we only double the seating capacity then it would be possible (if the percentage of seats occupied remained the same) then the average ticket price could halve.
Now obviously HS2 aren't going to cut all their tickets by 50%, however what it shows is for the lower priced tickets to stay the same whilst reducing by a bit more than 50% the highest priced tickets. As such £40 tickets probably wouldn't reduce in costs, however a £220 ticket may reduce to (say) £100 (assuming that you can double the number of people who pay each of those prices).
However that's working on the assumption that low cost things (like track access charges and energy use) increase by enough to offset the larger savings from the higher cost things (like staff costs and lease costs).
If there's areas where costs are likely to increase it'll be good to know, as currently the balance appears to be that reduced ticket prices should be viable site to the extra capacity and the resided costs on two of the large costs (leasing and staff costs).