Earlier quoted context omitted.
I’m not arguing with you here but it would be interesting to know what about tunnelling is fundamentally expensive. Things I can think of: it is slow, so you burn through a lot of wages. To compare with road building the only major road I can think of, built in my lifetime, was the M40. It is 90 miles long and took 4 years to build but I don’t know what the cost was. The channel tunnel is much smaller than a freeway…
London's Crossrail (Elizabeth Line) is probably a better example of the cost of a modern tunnelling project (at scale) [1] https://en.wikipedia.org/wiki/Crossrail
Less issues if there’s little down there to begin with. Although personally I don’t see why America can’t just build a train like a normal country.