What? The chunnel cost £26 billion in (2023 equivalent), and is 31 miles long. That's nearly a billion a mile. This is by no means cheap (yes yes cheap is a comparative term), tunnelling is absurdly expensive and not realistic.

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.