Most of the studies I have read chalk it up to two factors. First is changing project scope. The regulatory and political environment in the US means that projects often go through several planning changes, introduced in the middle of development, leading to wasted resources not just to overcome red tape, but executing on the plans that become obsolete.

The second that I have seen is codified labor specialization. This leads to many more workers on a site, where each has a niche role, but low workload.

Don't forget the abusive use of "expert consultants".

HS2 in the UK is about double per mile.

The US does have some real problems though, the built in america stuff kills projects like this where there isn't a US supply chain in place. And the US tends to have very high standards for "safety" for rail especially.

Keep in mind that you can't compare all projects like this. CAHSR total price includes trains, stations and (probably?) operational costs for x years. You often can't compare two projects on a like for like basis.

Yeah under the sea where nobody owns the land and the only residents don't have lungs to scream at you for ruining your beautiful view.

Building rail is a nightmare here as well because nimbys reign supreme.

Denmark has just started building the tunnel, though Danish salaries are higher anyway.

(Germany isn't much interested in the project.)

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

> Germany just built a tunnel under the sea to Denmark for a lot less $ per mile.

How did you come up with that? It seems that the tunnel is $450m euros per km, while SF to LA will be $80m per km.