A subterranean high speed system is totally out of question. The costs would unbelievable high. For a single line of such sort you probably could build an entire traditional high speed network.

But if you want to read about plans for a subterranean highspeed network, there actually exists one project. It is called "metroswiss". Its the sole plan that has been ever seriously thought about which would have been exclusively about subterrenean high speed lines (maglev trains in vacuum btw). The plan is to create a metro system for the entire country Switzerland that would turn the entire country more or less into a large city in terms of reachability.

The project has in my eyes pretty little chances of realisation, mostly because of the costs of course. But its not dead, there are people still working towards its realisation. Have a look at their homepage:
Swissmetro - Das Flugzeug ohne Flügel - Startseite


Reasons why this specific project is not the most realistic you will find of that sort is that the tunneling costs are limited through rather small diameters, the fact that Switzerland is quite small and very mountainous (that means conventional high speed needs lots of tunnels as well) and the fact that Switzerland is a very train focused country with traditionally very high ridership numbers.


But to get away from fantastic projects back to reality. Instead of dreaming of fantastic projects the US should start to build mature technology that is impressive enough. There are countless potential candidates for high speed lines in the US.