[iso1631]

London's 2 mile northern line extension costs about $1.7b, so doesnt sound out of whack. Berlin did extend the U5 about 1.4 miles for $600m but it's the right order of magnitude.

[cwhiz]

I was wrong. The latest expansion was $3.5 billion per mile.

https://www.nytimes.com/2017/12/28/nyregion/new-york-subway-...