The first six-axle variant of the new generation with new engines was the SD45, whose engine had 20 cylinders and delivered 3,600 hp. Basically, this was ahead of the competitor General Electric, since at that time they did not yet have six-axle locomotives with more than 3,000 hp on offer. Still, the big engine of EMD wasn't without problems. The crankshaft sometimes broke because the long engine block could twist under load. This was fixed over time, but many customers preferred to buy the next weaker variant instead. This was the SD40, released just a month after the SD45 and producing 3,000 hp from 16 cylinders. Overall, both achieved almost the same sales figures with 1,260 and 1,268 pieces. While the SD45 could only be sold in the USA, the SD40 was particularly successful in Canada with a total of 330 units, and another 72 went to Mexico. Four examples for Brazil were built for the local gauge of 1,600 mm.
Similar to the GP38, an SD38 without a turbocharger with 2,000 hp was also created. The largest buyer of the standard-gauge variant was Penn Central with 35 units, and 28 more were built for domestic and foreign customers. The Brazilian State Railways ordered 45 SD38M in the 5 ft 3 in broad gauge. An SD39 was also created from 1968, which had the smaller, supercharged V12 with 2,300 hp. Since the engine required less space and the bodies were therefore shorter, larger platforms were created at both ends of the locomotive with the same overall length. It was built 54 times, of which 26 went to the Southern Pacific and 20 to the Santa Fe. Derived from this, Milwaukee Road ordered ten SDL39s with the same engine in 1969, which were brought to the lowest possible total mass and thus had an axle load of only 18.9 tons. They were sold to the Chilean FEPASA in the late 1980s.
The SDP40 was offered for use with passenger trains as early as 1966. It got train heating, a gear ratio for 83 mph, different couplers and a braking system that, unlike the brakes otherwise used in the USA, could also be released incrementally. Because the SD40's frame was long enough to accommodate the additional equipment, the SDP40 stayed the same overall length even though the superstructure was longer. Eight units were sold to the Great Northern and twelve to the Mexican State Railways. Also available from 1967 was the SDP45, lengthened five feet to accommodate the larger engine and train heater at the same time. Launch customers were the Southern Pacific and the Great Northern with ten and eight engines respectively. A total of 34 more followed later for the Erie Lackawanna. The latter later only used the locomotives in local transport and equipped them with a shorter gear ratio and a different circuit of the traction motors to achieve better acceleration.
