Originally intended for use on branch lines, EMD introduced its Special Duty series in 1952 with the SD7. This loco also introduced the three-axle Flexi-Coil truck with its powered center axle. This was a major departure from earlier three-axle designs where an unpowered center axle was used to spread the engine's weight over more wheels, reducing the strain on the lighter rail and lower capacity bridges found on branches. EMD's design not only spread the weight but the extra traction motors provided a bit more power to get trains started. The SD's longer trucks required a longer frame, and buyers could also specify a heavier frame for engines working transfers and other low speed service. Somewhat underpowered with only 1500 horsepower (same as a four-axle GP7), only 188 SD7s were built from February of 1952 through November of 1953; the more powerful SD9 was introduced the following January.

* Prototype-Specific Detailing * Magnetic Knuckle Couplers * All Wheel Drive & Electrical Pick-Up * Dual Machined Brass Flywheels * Constant & Directional Headlights * RP-25 Wheels * Heavy Diecast Chassis * Five-Pole Skew-Wound Motor * Factory-Installed Sound & DCC Sound can be operated with standard DC power pack or DCC controllers. Sound features include: Diesel Engine, Horn, Bell, Squealing Brakes, Doppler Effect, Air Let Off (in neutral), Headlight, and Trailer which mutes whistle and bell for double heading.