Sol is, as usual, correct here; among great American cities, the only one with a location as strange as Atlanta is Dallas. The cities on the fall line in Georgia are Augusta, Macon, and Columbus.
One disadvantage that the fall line cities that far south have is that the weather is presumably pestilential, which would explain why Piedmont cities like Atlanta and Birmingham would have become relatively more important once the railroads made them possible. That doesn't explain why Atlanta got so much bigger than Birmingham, though. Instinctively I'd expect the opposite.
I-35 seems to follow a natural "minimum precipitation to sustain a city without industrial irrigation" climate gradient. I don't think Dallas (or Austin or San Antonio) is a coincidence.
