From what information I've found AHM's Helium Car appears based on the American Car & Foundry prototype built at the Berwick, Pennsylvania plant in the mid-1950s. Early production Helium Cars included 1930-built GATX models, as well as some produced by Magor. Though AHM offered the car decorated for folks like Wabash and NYC, apparently the cars were exclusive to the U.S. Navy (USNX), U.S. Department of the Interior's Bureau of Mines (MHAX) and U.S. Atomic Engery Commission (ATMX). By the 1980s, all cars are reported in Bureau of Mines service and by 1996 the fleet was out of service. I've seen them running on various railroads in the U.S., research suggests loads originated in the convergence of New Mexico, Oklahoma, and Texas Panhandle region served by Missouri Pacific, Santa Fe, and Rock Island. Loaded cars went to various U.S. government installations, including NASA's Cape Kennedy in Florida, Vandenberg AFB in California, Oak Ridge in Tennessee, and other points. Prototype information for the car may be found in Ed Kaminski's book "American Car & Foundry" by Signature Press. Information regarding Santa Fe's helium operations is included in Stephen & Cynthia Priest's "Santa Fe Heritage" series of books from Paired Rail Publications. Examples examined were built by Roco in Austria for AHM. Origin marks on the car's underframe include Austria in all caps and the small Roco logo of the time. Information researched suggestes the prototype length to be 45-feet, while AHM's model is 40-feet long. This short version may have been an economy attempt to allow for recycling an existing Roco underframe for this model.