[OceansOnPluto]

I can't speak too London but one reason that is for Paris is that the city is far, far smaller than New York. I think it's about 1/8th the size.

[rayiner]

The figure I'm referring to is per kilometer: https://www.nytimes.com/2017/12/28/nyregion/new-york-subway-...

[OceansOnPluto]

But you can't really isolate for kilometers because different structural considerations and infrastructure rerouting is considered based on the size of the city. Redoing the infrastructure to accomodate a subway is a hell of a lot more complicated the larger the city is.