2) Again, fast travel across the globe is a luxury, not a necessity. More than 80% of the world's population has never been near an airplane.
And that's for a single pair of cities, with no cost overruns, no tunnels, straight line distance. It also excludes things like stations, rolling stock, and maintenance.
It's not impossible to build, but having high speed rail throughout the US is an investment that would be a multiple of the costs of building the entire interstate highway system.
Mind you, I'd love it if that were to happen, riding high speed rail is much more pleasant than flying. Speaking as someone who has gone across the continent a few times on Amtrak and enjoyed riding high speed trains in Asia and Europe, it's just hard to see it happening. Recent US infrastructure projects have had massive cost overruns, and there is a seemingly low political appetite for building large infrastructure compared to the 20th century.
[0] https://www.statista.com/statistics/764486/cost-construction...
[1] https://www.wolframalpha.com/input/?i=distance+between+los+a...
Great, that's about 2 thirds of the yearly US defense budget! You have the workforce, the discipline, and as a bonus the defense industry can be repurposed for building rails and trains. Once the transition is achieved, you can build such a line every year at no added cost to the taxpayer.
(I know, that's completely unrealistic... it shows the priorities of the US government...)
But much of it is either for leisure or can be replaced by videoconferencing while saving money.
Regarding leisure, it might feel great to do fast intercontinental travel. But realistically, it is not something which will make you a happy person. Most people who are living today could not afford that luxury and are still living a happy life. Maybe even happier than the average American if their culture is a bit less materialistic.
In the light that it is going to devastate our planet, travel for leisure is not necessary at all.
Intercontinental and cross continental travel are a bit harder to replace with low carbon alternatives.
Europe has a functional high speed rail system due to its high population density, but the distances between some city pairs in the are hard to replace with rail or other low carbon alternatives. Think of Los Angeles to NYC, or San Francisco to Paris.
Many of these flights could be replaced by video conferencing, but if one really has to travel that distance, rail is a hard sell. It's not unpleasant to take Amtrak across the continent, but it's not fast either.