M-10004

The Union Pacific Railroad's M-10003, M-10004, M-10005, and M-10006 were four identical diesel-electric streamliner train 2-car power sets delivered in May, June, and July 1936 from Pullman-Standard, with prime movers from the Winton Engine division of General Motors Corporation and General Electric generators, control equipment and traction motors. One was for the City of San Francisco, two were for the City of Denver, and one was a spare set intended for both routes. In 1939, M-10004 was split and converted into additional boosters for the other sets, now renumbered CD-05, CD-06, and CD-07, all running on the City of Denver. The M-10001 power car became the other third booster. In this form, the three power sets ran until they were replaced by E8 locomotives in 1953, at which point they were scrapped.

The M-10003 through M-10006 represented the final development of the custom streamlined trainset on the Union Pacific. They followed the cab/booster unit concept pioneered with the M-10002 trainset. As totally separable and interchangeable cab/booster power sets, they set the path that EMC was to follow with introduction of their E series locomotive sets the following year. Union Pacific was able to maintain daily service on the Chicago-Denver run for seventeen years by dedicating three locomotive sets to that service and salvaging powerplants from M-10001 and M-10004 (three total) to provide additional power and keep at least two of the locomotive sets in running condition at any given time.

These power sets had stylistic elements in common with the Illinois Central's Green Diamond unit, which was completed just previous to them. Abandoning the "turret cab" styling of M-10000 through M-10002, these units adopted a new "automobile design" elevated cab, as with the Green Diamond, behind a longer nose than that of the Diamond. They shared a divided front air intake grille that dominated the nose, edged in shining stainless steel. Beneath, the pilot was edged and barred in stainless steel, very like the Diamond's. The Diamond kept the headlight at the top of the cab, while with these power sets a large headlight tipped the nose. The IC121 Diamond was a single power unit setup with a smaller articulated trainset, and in that regard had more in common with the earlier M-10001.

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