[luca_ing]

> I posit that for a city of 500,000 - 1,000,000 it might become practical to have subways and that radically changes the transit equation.

Cities of 500000 inhabitants which have a subway exist. See

https://en.wikipedia.org/wiki/N%C3%BCrnberg

[shadeless]

It's worth mentioning that the smallest city with a rapid transit system (2 metro lines, 28 stations) has a population of only 135,629.

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

[macNchz]

Lausanne recently took the smallest-city-with-a-subway title from Rennes, France (214k).

My family is from Lausanne and I spent a few months studying in Rennes–the size and density plus the metro system makes these cities really pleasingly 'human scaled' in my view. Walking-first center city areas with lots of restaurants/shops, easily accessed from less dense areas by metro+good bus systems+intercity train, but small enough overall to not feel sprawling.