After the launch of the GP7, customers expressed the desire for a six-axle variant with a lower axle load. The GP7 was equipped with two three-axle bogies while maintaining the same engine power and the frame was lengthened, resulting in the SD7. The abbreviation stood for “Special Duty”, but is now interpreted as “Standard Duty” due to the widespread use of the SD series. Despite the lower axle load and better traction, many customers shy away from the purchase due to the higher acquisition and maintenance costs, which is why only 188 were sold between 1951 and 1953.
Similar to the GP9, the SD9 was introduced in 1954, which also delivered 1,750 hp. 515 were built until 1959, 44 of which went abroad. Foreign customers were Korail from South Korea and mining companies from Chile and Venezuela. Shortly after the GP18, the SD18 was created, which in turn had the same prime mover. It was sold 114 times, and this time more than half was exported. 45 went to Brazil, while Korail bought another 15.
Even before the SD18, the SD24 was offered from 1958, which was the first EMD locomotive to use the mechanically assisted turbocharger. Since the six traction motors used could handle up to 400 hp, the output of the prime mover could be set to 2,400 hp. Although this did not fully utilize the engine's potential, full power could be accessed even on the highest passes in the Rocky Mountains. 179 SD24 were built, all going to US customers. In addition to its 30 SD24, the Union Pacific ordered 45 B-Units SD24B without a cab.
