The SD40-2 was introduced in January 1972 as part of EMD's "Dash 2" series, competing against the GE U30C and the ALCO Century 630. Although higher-horsepower locomotives were available, including EMD's own SD45-2, the reliability and versatility of the SD40-2 made it one of the best-selling models in EMD's history, edged only by the GP9, and the standard of the industry for several decades after its introduction. The SD40-2 was an improvement over the SD40, with modular electronic control systems similar to those of the experimental DDA40X.