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25 million Americans live in the 10 biggest cities.

33 million Americans live in the 20 biggest cities.

44 million Americans live in the 50 biggest cities.

I think any of those is a reasonable number to build a rail system for.

If you count by census statistical area, the 10 biggest MSAs contain ~80 million people. There's 180 million people in the top 50 metropolitan areas, well over half the USA, all living in conurbs of at least a million people.

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