Watch MoviestoWatch. Union Pacific's steam locomotive #3985 is the largest operating steam locomotive in the world. This 4-6-6-4 locomotive, built in 1943 still runs today as part of the railroad's public relations program. In this DVD, we'll follow 3985, which is also called the "Challenger", in Wyoming, Nebraska, Iowa, Missouri, Illinois, Oklahoma & Texas as she shows the public what big steam was really like.