[2] Union Pacific has marked DD40X on the cab exteriors, while EMD literature inconsistently refers to this model as either DD-40X or DDA40X.

Beginning in 1963, Union Pacific had ordered EMD DD35s and DD35As to replace the turbines, and the DDA40X was a further development from this design.

[4] The first DDA40X, UP 6900, was delivered in April 1969, in time to participate in the celebrations of the centennial anniversary of the completion of the first transcontinental railroad driving the "Gold Spike Limited"; it arrived in Salt Lake City, Utah, on the morning of May 10, 1969.

The X in the model number stood for experimental, as DDA40X Centennials were testbeds for technology that would go into future EMD products.

A few of the units were fitted with Federal Signal Thunderbolt air raid sirens to warn track-side personnel when away from grade crossings, but the results were inconclusive.

Despite their excellent performance and relatively good efficiency, these units were costly to maintain, which ultimately prompted Union Pacific to begin placing them into storage in the early 1980s.

But in early 1984, as rail traffic rebounded, Union Pacific brought 25 units out of storage and rebuilt them to return to service.

6915 was driven by Mike Watt as a drunk engineer, who threw bombs off of the locomotive and used it to run over pedestrians.

[14] In April 1995, the railroad successfully sued the video's producers, MTV, Sony Music, and Satellite Films, for copyright infringement.