The track access charges for HS rail are exponentially higher than classic rail. In Kent the whole county paid a premium 5% plus inflation on tickets for several years to pay for HS1. As a consequence we have some of the highest rail fares in the country but still need a considerably increased subsidy, that's with HS1 trains full and standing in the Peak (PRE Covid).
Just for clarification I want talking about HS2, only XC.
The issue with HS1 services is that the Southeastern services are short trains. HS2 services would likely have higher track access charges, other charges would likely not increase.
For instance having 16 coach trains rather than 9 or 11 wouldn't make a big difference to the lease costs and (assuming that at some future date the trains would need to be all 11 if not 12 coaches long) may actually result in a reduced lease cost.
Right now there's probably some who are thinking that how can having 16 coach trains be cheaper than 11 coach trains, right?
Well let me explain, it comes down to the increased speeds.
Currently it's 5 hours from leaving Euston to run to Manchester, to do the return trip and be ready to leave again. Under HS2 that stops to 3 hours.
Therefore 9+9+11 coach services every hour for 5 hours (before the first is ready to rerun the route) requires 145 coaches.
Change this to 16+16+16 for 3 hours requires 144 coaches.
The problem is that if we increase that to 11+11+9 that increases the number of coaches to 155, an all 11 coach fleet would be 165 and make it an all 12 coach fleet (one of the alternatives to HS2) and that's 180 coaches.
Now the savings aren't as clear cut on shorter routes (such as London Birmingham) with the existing fleet, however again as you increase the length of the existing fleet the savings get more noticeable.
However given that the HS2 trains have 1,100 seats vs a total of 1,058 seats when you total up the seats in both an 11 and 9 coach 390 (589+469) there's plenty of scope for extra passengers to cover any extra track access charges.
Anyway that's off topic from XC, but does highlight that longer trains don't always have to cost more than keeping the existing fleet. Whilst XC doesn't have the benefit of faster journey times like HS2 it would have scope for lots of extra passengers, especially if there's scope to reduce ticket prices a bit (as highlighted in my previous post).