"Why we're reaching our limits as a one-hour city"

Specifically, I was struck by this paragraph:

The one-hour-wide city, in Sydney, is reaching its limits. A city that has got 20 people a hectare and 40 kilometres an hour will become dysfunctional after about 2.5 million people.

One of the topics I'm very interested in is who gets rail--how big (and how dense) does a metro area have to be before it gets rail? (Or equivalent fixed-guideway transit).

My prior 'rule of thumb' has been about 2 million people. I would be VERY interested to see where (and how) the author arrives at that number. The author, Peter Newman, is a professor of Sustainability in Australia, who popularized the term 'automobile dependence' and write a highly cited book about it.