Similar to the four-axle models, EMD also offered the current six-axle models from 1972 in a revised form as „Dash-2”. Again, improvements included the electronics and the reliability and efficiency of the engines. In addition, new bogies were used, in which all traction motors were arranged in the same direction. This resulted in a greater overall length of the locomotives in order to be able to better accommodate the traction motors. 

At the lower end of the power scale was the SD38-2, which like the SD38 and the current GP38-2 was equipped with a 16-cylinder without turbocharger with 2,000 hp. As with the other 16-cylinder variant, a noticeable feature was the relatively short hood compared to the longer frames of the locomotives, which resulted in very large platforms for the switchman at both ends. As with its predecessor, it became apparent that a six-axle diesel locomotive with such a low output should not achieve large sales figures. Thus, a total of only 90 examples could be sold, which were distributed among many buyers.

The most powerful model was the SD45-2 with the well-known and not entirely undisputed 20-cylinder with 3,600 hp. The fact that it did not come close to the sales figures of the SD45 could be linked to the bad experience with the overly long crankshaft in the SD45. Only 136 of the original SD45-2 were sold between 1972 and 1974, of which 90 went to the Atchinson, Topeka and Santa Fe Railway. A further 247 examples were built of a variant called the SD45T-2, which was also dubbed "tunnel motors". The difference was a different design of the cooling system, which had been adapted for use in the mountains with many tunnels and snow sheds. Due to the fact that the fans on the roof were often covered, the inlets for the cooling air were moved downwards at the level of the running boards. These machines all originally went to the Southern Pacific and the St. Louis Southwestern.

Kansas City Southern SD45T-2 with special radiator fan grills

Terry Cantrell

By far the most successful variant of the Dash 2 family and overall one of the most successful locomotives from EMD was the SD40-2 with 3,000 hp. Very large numbers were built, including 835 for the Burlington Northern, 686 for the Union Pacific and 486 for the Canadian Pacific. It wasn't until the early 1980's that sales in the US dropped due to competing models like the SD50. In Canada and Mexico, the popularity remained unbroken for some years, so that a total of 4,175 pieces had been made by 1989. There were also 312 pieces of the SD40T-2, which had the same adjustments to the cooling system as the SD45T-2 and of which 229 engines went to the Southern Pacific. Derivatives of the SD40-2 for export came in various variants of the JT26 and GT26, including the British Class 59.

The large number of SD40-2s was subject to many rebuilds over the years. Some locomotives modernized since 2010 are referred to by their operators as SD40-3. These include the CSX machines, which, thanks to modern electronics, develop around 50 percent more starting tractive effort with the same engine power. The Brazilian Ferrovia Central Atlantico had some used locomotives converted to meter gauge, each with four two-axle bogies, which are called BB40-2. Other locomotives later received more modern engines that meet stricter emission standards. Among them is the SD22ECO with a V8 with just 2,150 hp, which is used on feeder routes. Some SD40-2s that were involved in an accident were rebuilt without a driver's cab and were thus converted to driverless booster units. Other locomotives were converted to the status of the SD40-2, for example by installing the less problematic 16-cylinder engine in some examples of the SD45 or SD45-2.

A factory-built variant of the SD40-2 was the SDP40F for use with passenger trains. It was conceived from the fact that Amtrak, which was founded in 1971, had received a large number of cars from their former owners, but the locomotives mostly stayed with their owners and Amtrak therefore urgently needed new locomotives. Thus, the SD40-2 was provided with wider hoods analogous to the FP45 and two steam boilers at the rear. Frequent derailments soon happened, which could be attributed to the unfavorable placement of the large water tanks for the train heating in combination with the low weight of the baggage cars after the locomotive. Thus, after 150 units had been built, a switch was made to other types and some SDP40F were updated to provide head-end power with the availability of electrically heated passenger cars. Some SDP40Fs were exchanged with the Santa Fé for less powerful engines because Amtrak had a need for them.