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…
The most expensive parts of tunnels are where you come to the surface. The Channel Tunnel is cheap in that regards because there is countryside on either end and no exits in the middle. Designing a high capacity train station or highway exit in the middle of a city is quite expensive.
It's impossible to make the case for thousands of miles of underground tunnels that will add no benefit (what happens when something goes wrong? you can't simply overtake when something inevitably goes wrong (three train derailments per day in the US), it's underground so it's hard to access / fix problems etc etc.
It's a foolish, expensive idea, when the US struggles to take care of it's own railway systems.