I personally think Auto Trains outside of the one we already have could work, but it would probably be seasonal for the most part and would need to be run as a separate train. I do think the Zephyr could sustain a summer season Auto Train with stops in Northern California, Salt Lake City, Denver, Chicago and possibly Western Nevada. There are plenty of national parks along this route and others that people might want to take a car along with them. But, I do think this would be a largely seasonal market since the summer would be the best time to do this and would be the better market for it. You could schedule an Auto Zephyr to run through some places at night so people have the day to see parks in Utah and Colorado and possibly catch some of the more scenic parts of the route before they get into either city. But theses trains would need to run separately because like with the express freight thing, it takes time to cut cars and that already ruined trains reliability back in the 90s. Couple this with getting into Salt Lake City at 3am or 11pm or whatever inconvenient time in the middle would mean these trains would likely only be good for a few stops along any given route instead of all of them. This also doesn't take into consideration what you would do with any of the facilities during the winter when a train might not be running, but that is a different can of worms which would make more auto trains less feasibly.
I do think more auto trains could be feasible under the current model. Chicago - Florida has been pointed out, but I think Chicago - Southwest could also work for the same reason. A lot of snowbirds also go west too and Arizona and New Mexico are destinations they go to. For the same reason, I could see a Starlight to Sunset route auto train maybe working. But it would require more facilities, more equipment and an extra train on applicable routes. It would be far easier to get an Enterprise office next to every station in a town with 10,000+ people than expanding auto trains.