After the Union Pacific M-10000 to M-10002 had been built as different individual trainsets, the M-10004 to M-10006 and the M-10003, which was delivered later, were the first to be built as several identical streamlined multiple units. The M-10004 was intended for the “City of San Francisco” between Chicago and San Francisco, while the M-10005 and M-10006 were used for the “City of Denver” between Chicago and Denver.

Each set initially consisted of a power car with a cab, another booster, nine passenger cars and a lounge car with a rounded rear. The front design deviated from the tower-like cab of the predecessors and followed the carbody design, which later diesel locomotives also picked up. The nose was dominated by a very large grille and a snow plow, which showed a lot of chrome. The unit of power car and booster, numbered M-10003, was eventually built as a reserve to replace failed units.

In each power car and booster was a 1,200 hp Winton 201, which was later also used in EMC's E-series passenger locomotives. The generator, the electrics and the four traction motors each came from General Electric. The associated passenger cars were each designed with a low profile to match the powered cars. A total of six of the cars were connected in pairs to Jakobs bogies. 

M-10004 was exchanged for a locomotive-hauled train in 1939. The power cars and the already retired M-10001 were used to reinforce the other three trains by a third unit each. This increased the power per train from 2,400 to 3,600 hp, which also allowed additional cars to be attached. They were used in this combination until 1953 and then replaced with conventional trains with E-series locomotives.