This is a Minuteman I Transporter-Erector-Loader (TEL), on display at the Hill Aerospace Museum. The TEL, made by Boeing, was designed to transport Boeing LGM-30A Minuteman ICBMs.
It is 65 feet long and 10 feet wide, and has 8,030 cubic feet of interior space. When empty, it weighs 24,700 pounds. It was pulled by a custom 1963 GMC tractor. It was also possible to transport a Minuteman missile by rail without the tractor.