I believe Amtrak has never had a train connecting ATL and Florida (the Floridian went through Alabama). The last train I am aware of between the cities was the Dixie Flagler. If I had to start from scratch and build a single rail line between the NEC and Florida I would run it via RGH, CLT, and ATL. It would be longer than going along the Atlantic Coast but you'd have a much larger potential audience. Was there ever a train from the NEC to Florida via ATL (maybe Seaboard Coast Line)? How about a train between CLT and Florida?
Nothing by SAL/ACL ran from the NEC to FL via Atlanta. The SAL had a train NEC-ATL-Birmingham. If you consult your rail atlas you'll see that the SAL/ACL really didn't have any real routings possible for it.
Now the Southern (SOU) Railway might have. But I'm pretty sure their Florida service which was very limited went on other routes. I believe their only contribution NEC to FL ran via Columbia and what became the SAL.