The "F-unit" refers to a series of diesel-electric locomotives built by the Electro-Motive Division of General Motors from the late 1930s to the early 1960s. These locomotives were designed for both freight and passenger service and are known for their distinctive streamlined design. The F-unit series includes several models, such as the F3, F7, and F9, which were widely used across North America. F-units were significant in the transition from steam to diesel power in the railroad industry. Their reliability and performance made them popular among various railroads, including Santa Fe and Union Pacific. Today, many F-units are preserved in museums and by private collectors, showcasing their historical importance.