I guess MARTA is between municipal and regional rail, sort of like BART. I meant for connecting the Atlanta metropolitan area to its region, not intraconnecting.
Roughly speaking the density of the US east of the Mississipi and the far west coast is dense enough. The rocky mountains and Alaska really bring our average density down.
Depending on density. I'd wager the Atlanta metropolitan area is just about crossing that threshold, i.e. they should built it now.