Not sure about that, but it could have been an indicator of stations that offered car rental services.This is only loosely related, but I was looking through some old timetables for the Wabash Railroad and I noticed they advertised (without any detail) “Auto Service” on several of their passenger routes. Most of their routes were in the Midwest, including St. Louis to Detroit.
I’m guessing they put an auto carrier rack on the standard passenger train, but I have no way of knowing to be sure.
I’ve looked for details elsewhere on that service, but haven't come up with anything yet.
However, besides the example NS VIA Fan showed, IIRC, B&O/C&O in the Paul Reistrup era offered a similar service...again, IIRC they called it something like "Car-Go-Rail"....not sure...