Location, location, location.
Atlanta was a great location for a rail hub, regardless of the “mountains”. It is surprisingly close to Ohio, Illinois, etc. as well as centrally located in the Southeast.
Many people don’t really understand how “tilted” the east coast of the US is. A very large percentage of the US population is less than 600 miles from Atlanta.
Air travel made it just that much easier for these short distances to pay off. Throw in Atlanta’s head start over other cities like Chattanooga and its less provincial mentality and it really took off.
Note also that South America and the Caribbean is south and east of Atlanta, and you can easily see how it can add being a major player in connections to those areas.
Memphis, the site of the FedEx hub, is in a similar situation. But there, it (was) just one company picking a spot and being a less than busy site (at the time) plus being a little further west helped. Now a lot of companies have put up warehouses and such there so they can cheaply send stuff by air.
You may fly thru Atlanta, but your Amazon order might go thru Memphis.