In general, the longer your stay, the more economic sense it makes to take the train. When people take their own car with them, they are dealing with a comfortable, known vehicle instead of a rental that may or may not be what you really like.
If a new Auto Train could be operated on the Rathole south from Cincinnati, I'd be right there to ride it. Unfortunately, this would require cooperation from Norfolk Southern, who probably would be pretty determined to resist it. Getting a good routing from the Rathole to Florida might also be a problem.
Tom