40dd

The EMD DDA40X is a 6,600 hp (4,943 kW) D-D locomotive, built by EMD from 1969 to 1971 exclusively for the Union Pacific Railroad. It is the most powerful diesel-electric locomotive model ever built on a single frame, having two 16-645E3A diesel prime movers. Union Pacific has marked DD40X on the cab exteriors, while EMD literature inconsistently refers to this model as either DD-40X or DDA40X.UP's DDA40X locomotives were the ultimate culmination of the company's experiments with extremely powerful locomotives that began with its gas turbine-electric locomotives and DD35s. For manufacturer EMD, the construction of the world's most powerful single frame locomotive was a sign of the company's dominance of the North American diesel locomotive market, with only GE Transportation an equal competitor. The DDA40X also pioneered a number of new technologies that would go on to be incorporated in future EMD designs.
Ultimately, UP did not continue with exceptionally powerful locomotives like the DDA40X built on single frames, instead moving towards distributed power using smaller locomotives closer to 4,000 hp. All of DDA40X locomotives were retired between 1984 and 1986; several of them survive. Union Pacific 6936 operated as a member of the Union Pacific Heritage Fleet until 2022, when it was announced it would be donated to the Railroading Heritage of Midwest America museum.

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