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The BeltLine doesn't extend into Atlantic Station. The current finished portion of the Eastside Trail, which is probably what you're interested in, runs from Monroe Drive to Irwin Street. (There are other finished segments, but they are less touristy and, IMO, less interesting.)
The advice you got from /u/rickandmorty4ever100, to download the BeltLine app, is solid. Those links are:
If you're not in your own car, I also second the advice that you start at Piedmont and Monroe -- but if you're driving, I think you'll do better to park at Ponce City Market. If you do this, I suggest you also download the Parkmobile app. This will let you park for the shortest (cheapest) time window you like, and then extend it from your phone if you're having fun. That app is at:
You'll probably find a desirable lunch in Ponce City Market -- but if you're really into the sandwich idea you mentioned, Parish Restaurant (on the BeltLine between Ponce City Market and Irwin Street) might be more to your liking.
If you do end up starting at Ponce City Market, which is in the middle, I recommend walking south first until the trail ends. It's subjective -- but I think that section of the trail is more interesting and more attractive. Definitely see the Historic Fourth Ward Park, which is connected to the trail, but not adjacent to it.
A few pro tips if you download the Parkmobile app: