There is an effort underway to generate support for completing the unfinished and unfunded three miles of Silver Comet Trail which will connect the trail to the City of Atlanta, in particular to the trails which connect to the Beltline.
The Cobb Bicycle and Pedestrian Plan outlines three possible routes for accomplishing this. In my opinion, and in the opinion of the River Line Historic Area, by far the best route is an unused section of CSX track which goes from the current start of the trail near the interesection of South Cobb Drive and the East West Connector, to the river crossing near South Atlanta Road (which becomes Marietta Boulevard when it crosses into the City of Atlanta).
In 2010 this route was removed from the table because CSX thought they might activate the track. Since that time Georgia Power's Plant Atkinson has switched from coal to natural gas, diminishing the need for possible use of rail along that route, and CSX allowed Cobb County to pave over the tracks at Oakdale Road, a clear indication that they aren't planning anything for the tracks at this time.
Last Saturday the River Line Historic Area (RLHA) organized a walk of the unused tracks. Here is a link to photos from that walk.
At this point River Line Historic Area, Bike Cobb, and the Mableton Improvement Coalition are committed to advocating for this project. When it's complete there will be continuous greenway trails from Atlanta's Beltline to Anniston Alabama.