Originally Posted by BOSTransplant

Not the person you quoted, but I compulsively investigate things like this, and I've connected through ATL enough times to have that airport down to a science. Note, the below is assuming normal walking pace. Obviously it's possible to shed a few second by sprinting, but I've fortunately only had to do that once. Also, I've never timed it exactly, but I believe the distance from T Gates/Security Checkpoint to Terminal A is a bit longer than the distance from A to B. It's not a significant enough difference to factor into my calculations though.

At peak times, a new Plane Train arrives about 54 seconds after the last one pulls out of the station. If you're walking only one terminal (e.g. A-B, A-T) and you've just missed the closing doors on a train, then I've found walking is almost equal to waiting. I walk at a pretty brisk pace and use the moving sidewalks, and I still almost always arrive at the next concourse just as the next train is opening its doors there. If you have to go more than one concourse, or you have to go to Domestic Baggage Claim (recall that it is a different stop than T gates), then it is always faster to wait for a train.

At non-peak times, the train slows down to once every two minutes. Then, if you are going exactly one terminal and it will be more than a minute until the next one, it's generally faster to walk. If you're going more than one terminal, it's faster to wait, even if the wait is the full two minutes.