ATL is one of the easiest U.S. hubs for airside transfers and one of the easiest to underestimate. The Plane Train makes concourse movement fast, but leaving the airport means rebuilding time for MARTA, security, and boarding.
What to do with bags
Plan as if ATL does not have public luggage lockers for layover storage. If your bag is checked through, keep it that way. If you have a carry-on, ask your airline whether early bag check is available for your onward flight, or choose a bag-friendly plan.
Leaving the airport
MARTA is the cleanest city option from the Domestic Terminal. Downtown and Midtown are realistic with a longer layover; anything requiring multiple stops or rideshare traffic needs a larger buffer.
Getting back through security
ATL states ticketed passengers can access concourses from checkpoints, with hours varying by checkpoint. Keep your onward boarding pass and acceptable ID ready, and return early during morning peaks.