Service with Union Pacific

Built in January 1971, the UP classifies No. 6936 as a DD40X rather than a DDA40X as indicated by the model designation on the cab of the locomotive.^[1][2] It served in revenue service over the next decade and into the mid-1980s across the Union Pacific system. UP 6936 made its last regular freight trip on May 6, 1985. It went on to become a part of the railroad's excursion fleet.

The locomotive's last excursion with UP was in 2015, when it helped the UP's trio of EMD E9 units at the annual Cheyenne Frontier Days event. It was last used on July 12, 2016, when it helped to test UP 844, a steam locomotive being returned to service. Its dynamic brakes were used to simulate a load while being pulled.^[3] After the tests, it was placed into storage at the roundhouse in Cheyenne.

At the time, the current operating condition of No. 6936 was unclear. As of 2017, a page posted on UP's website said the locomotive "now travels on occasion as part of the UP Heritage Fleet."^[4] But a page posted in 2019 did not describe No. 6936 as being operational.^[5]

Acquisition by RRHMA

In April 2022, Union Pacific announced that 6936—along with Union Pacific 3985, Union Pacific 5511, and several other pieces of equipment stored at the Cheyenne roundhouse—were to be donated to the Railroading Heritage of Midwest America, with plans to return the locomotive to operation.^[7] In November of that same year, UP moved 6936 and the rest of the donated equipment to the RRHMA's large shop facility in Silvis, Illinois.^[8] As of 2023, No. 6936 is undergoing inspection and repairs to for a future return to operation.^[9][10][11] It made a successful test run on August 19, 2023.^[12][13]