The work on the extension of the Silver Comet Trail eastward toward the City of Atlanta is seriously underway at this point, and the new section of trail that will extend the SCT from the current start on Mavell Road (near the Publix on South Cobb Drive) to Plant Atkinson Road. That puts the endpoint very close to the Atlanta city limit at the Chattahoochee River.
This section of trail is projected to open in the summer of next year, and the long-term goal is to connect the SCT with the Beltline trails.
Read about the current state of the work here.