Technology

I'd personally opt for something more "conventional", like the SCMaglev being developed in Japan (or better yet, that Chengdu Maglev prototype). As fast as this train would go inside a vacuum tube, the tube itself will just add much more complexities and points of failure, on top of being more expensive to build/maintain.

Also a large part of the "experience" is also being to watch the scenery pass by at 300-600km/h+ out of the window, which is lost when inside of a very long concrete/steel tube.