Rail is best used for freight.
Autonomous busses and cars (electric) are the best long-term solution for travel.
The best approach is simply to enact a Carbon Tax and let the market figure it out.
We would be better off promoting mass-contraception to the developing world and halting land clearance in terms of massive targeted CO2 reductions.
You don't even necessarily need special high speed rail, just more rail makes it more high speed because you can mix slow and fast traffic (e.g. no-stop direct trains can run side by side with freight and services that makes 10 stops).
Speed doesn't have to come up so much to at least beat road traffic.
"...let the market figure it out" would work in a market that isn't already so strongly shaped by subsidies, regulations, and nationalist protectionism.
Autonomous vehicles will have a very large and currently underestimated impact on all kinds of other modes of transportation, but there is a wide range of expert views on when that will happen.
I think high speed rail could be potentially a game changer for the job market. In the UK if you could easily commute in from Birmingham or Newcastle then London salaries would be more accessible outside London.