M-10000 was the first of 11 streamlined “M” units delivered to Union Pacific in the 1930s and 1940s. The “M” stood for “motor,” and designated locomotives that were powered by internal combustion. This first streamliner, made of aluminum alloy, was a three-car day train; sleeping cars were added in subsequent models. Fully loaded with 116 passengers, crew and baggage, M-10000 weighed far less than a conventional steam train and used less horsepower. It was nicknamed “Lil’ Zip” and was recognizable by its modern shape and vibrant Armour Yellow paint job.