Since the gas turbine locomotives built for the Union Pacific in the 1950s did not manage to establish themselves as standard locomotives for very heavy trains, the development of particularly powerful diesel locomotives was tackled again in the 1960s. However, the power of the single prime movers was limited, so it was decided to combine the power plants of two locomotives each on a common frame. Thus, there were two 16-cylinder series 567A engines, each on a frame with its own generator, which in turn stood on two four-axle bogies.
The first model was the DD35, which was built 30 times in 1963 and 1964, mainly for the Union Pacific. Their prime movers were identical to those of a GP35, giving a combined output of 5,000 hp. They did not have a cab and were intended for use as a B unit together with classic diesel locomotives. Since the four-axle trucks at the front of the train greatly reduced the service life of the rails, no A-units with a cab were initially built. Usually two DD35 were used between two GP35, giving a total output of 15,000 hp. It was not until 1965 that 15 DD35A cab-units followed, built exclusively for the Union Pacific. Both types had in common that the sandboxes were initially in the engine room and sand flying around caused problems in the electrical system there, which led to the sandboxes being relocated to the running boards.
The high point of the development of the eight-axle EMD locomotives were the DDA40X “Centennial”, of which 47 were built between 1969 and 1971 for the Union Pacific. New 16-cylinder series 645 prime movers were now used, which brought the total output to 6,600 hp. Thanks to further developments on the trucks, it was no longer a problem if such a locomotive led the train. On the inside, many parts which later found their use in other EMD locomotives were used for the first time. The cab was of the wide design used on the FP45.
In day-to-day operation, however, it soon became apparent that such a powerful locomotive hardly brought any operational advantages compared to two smaller ones and was less flexible to use. For this reason, no further eight-axle large diesel locomotives were later developed, apart from special narrow-gauge types with low axle loads. All later models had only four or six axles and were or are mostly used at least double-headed. Even today, when the current standard diesel locomotives have an output of more than 4,000 hp, trains with ten locomotives are formed on mountain lines. Also the DD35 and DD35A were retired in the early 1980s and the DDA40Xs in the middle of the decade.
