As a snowbird in a group that has a large Midwest contingent, the destination of choice is the Northwest Florida panhandle, near the Alabama border. That's a full day's drive from the southern terminus of the current Auto Train. Unlike the disaster that is I-95, our group members primarily use I-65, which is relatively flat, seldom busy and less impacted by severe winter weather than the more hilly Eastern options. Therefore it just makes sense to drive one's own vehicle, and parking lots between Panama City and Mobile are literally full of license plates from the Midwest states, Ontario and Manitoba. Not sure where the top end of an Auto Train "funnel" would have to be to capture the most users, but not sure the present southern terminus in Sanford would be that practical either.