These Union Pacific Class F-50-11 flat cars were built in early 1941 as part of series 57000-57299 and feature the M3 Lee tank Load. The M3 Lee was an American medium tank used during World War II. Design commenced in July 1940, and the first M3's were operational in late 1941. This was to meet the need of the US Army to have s medium tank armed with a 75 mm gun and coupled with the United Kingdom's immediate demand for 3,650 medium tanks. The design was a compromise meant to produce a tank as soon as possible and serve only until replaced by the following M4 Sherman tank.