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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 — 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.

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You could calculate the cost to tunnel under one linear mile at 40 ft diameter and then realize that whatever number you got, it is the cheap part because you haven’t reinforced, drained, ventilated, or surfaced the tunnel.

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.

It's even easier than walking through that. Just Google the costs per mile of a typical subway line extension. There's a reason subterranean rails only makes when land costs are at a premium.

Even then most subways would be better as elevated rail

That adds a huge amount of clutter to the surface, and is very noisy for anyone nearby.

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

Keep in mind it also has to navigate its way around what’s under London which is a lot more complex than most places, considering it’s full of tunnels, Victorian sewers and mass burial pits.

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.

Indeed, if there are administrative overheads of constructing in built up areas then it would be important to compare with a major highway building project also done in or near a built up area.

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.

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.

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.

The more modern tunnel project in Seattle, USA would be useful to compare (instead of the Chunnel).

https://en.wikipedia.org/wiki/State_Route_99_tunnel

A road is just 2d tunnel.

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