A few hilarious links:

http://www.uic.org/cdrom/2006/wcrr2006/pdf/388.pdf

https://www.researchgate.net/publication/37449974_Global_modelisation_of_the_Swissmetro_maglev_using _a_numerical_platform

Key idea: social and economic networks are limited in the end by travel times. Planes are fast, but can't land in city centers, and getting on one is a major production.

Planes are fast because they can go high, where air resistance is less because there is less air.

If you run your trains in a tunnel, and seal the tunnel properly, you can pump most of the air out of the tunnel and run trains faster than airplanes. And keep the whole city center stations, precise schedules and rapid on-off boarding paradigm.

The Japanese proposal is considering 900 KM/h maglevs.  This how you do nationbuilding by engineering fiat - once the tunnels are done and the tracks are laid, the places connected will be so close in terms of travel time that untangling them politically becomes a very dicy proposition.
Also, of course, some military utility to the ability to redeploy forces at 900km/h underground.