According to Robert Whiteacre's book, "Marx Trains, Volume 3, Sets", there was a 1956 set no. 52975 that was comprised of an olive 2-4-2 diecast #666 locomotive, a 1951 US Army olive plastic tender, and some eight-wheeled cars: a short, olive tank car with a gray tank as load; a 2824 flatcar US Army olive missile launcher; a 2236 US Army olive gondola, and a 234 US Army olive caboose. That set sounds like the one you have.
Prior sets include other types of cars, such as 2246 flatcars with vehicle loads, 2130 work caboose with tank, a 2858 US Army olive boxcar.
Maybe some others will know of other eight-wheel cars.
