The problem is - Let's look at Japan - it's average Population density is 343 people/sq KM - which basically means that except for the most rural parts of Japan, it pays to run commuter trains

Now, I just picked a Midwestern state - Wisconsin - it's population density? 38/sq km

Ohio - 107

The list goes on.

What it comes down to - look around at the US big cities and their suburbs - they DO Have Mass Transit - ot a lot of them do. NYC (Yep - The MTA and it's subsidiaries), Chicago, Yes, LA, Boston, Philly etc - all yes. There are citys that could benefit from the addition of Mass transit (Hello Texas and Florida)

The problem is, even when you get to exurban NYC - say you're getting out a ways on some of the NJ Transit lines - of for that matter, most of them, they don't really pay, and the only reason there is service out that far is that the rail lines are there for freight use

Now, interesting, I take mass transit (disclaimer - for the last 2 months, I have not due to a leg injury, and the inability to walk even the length of a train platform - but since 1992, other than that...). I like Mass transit. A friends father used to help design mass transit systems. It's just that for it to be at ALL reasonable, you have to have N number of people within a certain distance of the right of way, otherwise it does not "pay off" even with a subsidy - and you can look at that as even on a polution level - you generate X amount building the line, Y amount maintaining it, and Z amount running the trains - it can get rural enough that you'll never pay back X+Y+Z

Like I said, I forget the exact population density breakpoint is, but it IS there