This doesn’t require a discussion to figure out. You just have to mentally walk through the process and do some back of the envelope math.
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.
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.
I’m still left wondering though, in the open countryside, what the complexity (and cost) comparison is between laying down a mile of freeway and boring out a mile of tunnel. It feels like tunnelling is hard and complicated, but so too is making millions of magic coloured tiles that are all connected to each other so we can see each others cats and, very rarely, make phone calls.
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 — two train tunnels and an escape tunnel in between. It is 30 miles long, took 6 years to build, for £9bn at the time.
Tunnelling also must require a lot of energy. What it probably doesn’t lean heavily into, in terms of cost, is either compulsory purchasing of the majority of the route or the political consultation and planning process. Those were both a huge deal when building a motorway through the Oxfordshire countryside.