This has been pounded to death in other threads, but the conclusion I have come to is that for the Auto Train to work, you have to have a route short enough to make daily non-stop round trips with enough time for basic maintenance, loading and unloading at either end, and schedule pad to be successful. In addition, you have to place that defined trip time (mileage is not as important as is a guaranteed trip time) in between MAJOR human migratory routes. That's why the Auto Train today works - it's a 17 hour trip between the heavily populated NE and the heavily populated SE. Really, the existing terminii are just about perfect. From Sanford, you have one hour access to Orlando or Daytona, you have 3 hour access to Tampa and about 5 hours to go to get to Miami. If the Northern terminus could be brought up closer to Baltimore or Newark, I think that ridership would increase dramatically, but it would be impossible to make the trip with two trainsets.
So, that all being said, is Louisville a practical terminus for another Auto Train route? I don't think that the flow of people from Chicago/ Indianapolis/ Cincinnati to the SE even comes close to the migratory pattern of those travelling from NY, NY, et al. Rader seemed to think there was 20 years ago, and who knows? Perhaps if the track conditions were NEC quality from Louisville to Sanford, then it could happen. But Auto Train can't afford to upgrade the Freight's track for that level of required service.
Where else do such migratory patterns occur? I think the only other is between Chicago and NY. But that's a lot shorter route than Lorton to Sanford, and there are so many more places in between that makes frequent stops more desireable than non stop Chicago to NY daily for 400 pax and their cars. Perhaps if it were run as a day train, that would make a difference.
I don't know if California - Washington State has that level of migratory activity either. Both are rather temperate year 'round. I don't know of a "Snow Bird" pattern of Washingtonians fleeing to Southern California for the Winter. I could be wrong, though, since I've never really assessed that market.
Just my 2 cents worth.