Thanks for the comments! After spending part of my career in transportation construction, and now trying to write about "futuristic" travel, I have these observations:

1) Tunnels will probably always have high costs, which tends to make highway medians better conveyances for mass transit. Otherwise, right of way costs are astronomical/time consuming, which does favor tunnels somewhat (which justifies the one under downtown Seattle, for instance). In any case, for trips of >300 miles length, how can anything compete with air?

2) In construction, the adage might be "if you can possibly precast, precast" (due to the issues of mixing/casting concrete onsite). Tunnels encounter different geologies with varying reinforcing requirements. I keep waiting for some kind of super cement that can employ onsite soils (provided it has requisite sand/gravel)

3) Thanks for the update on the Boring Company.