- Figuring out NIMBY-ism. Anywhere you run a tunnel you're gonna have people suing you and stalling for decades. Less so if you use a tunnel bore machine, but cut and cover is pretty much a non-starter.

- Cost of labor is insanely high due to cost of housing. Short of jumping straight back into the 19th century, setting up temporary housing and bringing in guest laborers this is pretty much non-negotiable.

- Not a ton of expertise left in the country since there's 2 new subway tunnels a decade AFAIK.

- The grift has got to be worse here than in Spain. There if you get $40k in kickbacks that's a nice bonus, here that barely covers your rent for the year.

And then even if you bring the costs down, you have to figure out the taxation. Several billion per mile is the running rate and you may be able to bring that down but then you have ongoing costs. Muni's farebox recovery is only 1/4 of its budget so unless you're making existing lines redundant, there's new ongoing cost. Obviously the choices there will be to go into the pockets of the middle class or not do it at all.

> Cost of labor is insanely high due to cost of housing

This is not the reason. Labor is expensive even in parts of the US with low housing costs.

The real, simple reason is the US has a more prosperous economy where the average worker has more opportunity than their Spanish peers. Just look at unemployment rates. The US is at 4.3% right now compared to Spain's 10%. Even at the peak of the GFC the US barely had over 10% unemployment. In the meanwhile Spain has had over 10% employment almost the entire time the past four decades. Of course labor is cheap when that many people are jobless.