Several documents on a proposed HSR line in England have recently been released after a Freedom of Information Act Request. Links to these are below:
“Estimated Carbon Impact of a New North-South Line” (pdf)

1.20 (p.6) The London to Manchester base scenario results indicate that none of the rail
options under consideration achieve emissions parity, even at 100% rail share. In
other words if a new line is constructed and operated on this route, regardless of the
rail technology employed, the amount of emissions generated would not reduce to
the level emitted in the do-nothing scenario. Therefore, based on the assumptions
applied, there is no potential carbon benefit in building a new line on the London to
Manchester route over the 60 year appraisal period. In essence, the additional
carbon emitted by building and operating a new rail route is larger than the entire
quantity of carbon emitted by the air services.
1.21 Figure 1.3 illustrates the key findings for the London to Glasgow/Edinburgh route
for the base scenario. The results are substantially different than those for the
London to Manchester route, showing how emissions parity can be achieved for all
rail options, at increasing levels of rail share.

DfT New Line Capacity
Study – cost estimate
This document is redacted, i.e. key numbers are missing, because “The release of this information has the potential for disproportionate and unwarranted adverse impact on property values which may result from publication (generalised blight).” but for the HSR analyst, there is still lots of good comparable information on other systems.
The reports were prepared by Booz, Allen, and Hamilton consultants.