Yes, it is real. According to Hiteshaw's excellent 6" tin guide (highly recommended!) they were made from 1953-1962. Plastic wheels went into production around 1949. This would have been part of a windup set, with the sliding slot & tab couplers. I had to go grab my example off the shelf to look at... it's one of my runners I use on the clockwork layout.
If you are interested in Marx 6" tin, Walt Hiteshaw's CD is THE reference to get... worth every penny: http://www.toyandtrainguides.com/
Nice car John, congrats on the find!