That's a good list! Leave it to Wikipedia junkies to compile this stuff ...
It's ranked, by default, by density. Find Paris: 32nd on that list. Now search up from there to find cities that have more than 2.1M people. You'll find only 4, all of them in India.
That is: you can find plenty of places that are denser, but they are almost all smaller. Union City, up here in NorCal: 39th in density, but tiny: 62k people, so like who cares?
My point: France is different, and Paris more so. This combination of density and size makes it an awesome market for rail.
/jordan