With a new Dash 2 electronic control system and high traction trucks, the SD40-2 was unveiled to railroads in 1972, and quickly became an industry favorite. Nearly 4,000 were built, making it the single most popular diesel model to date. After 30 years of service, many are still working for their original owners or merger partners and various leasing companies. Over time, the prototypes underwent production modifications, and owners made additional equipment changes. Models represent the original version.