All were the mechanical ancestors to EMD's successful E-units, with identical pairs of 900 hp (670 kW) Winton 201-A Diesel engines, although they ran on AAR type B two-axle trucks instead of the A1A trucks of E-units. When delivered, the units were fitted with shrouding around their trucks, but this did not last long.
The boxy carbodies of all but the Zephyrs were the work of GE's Erie, Pennsylvania works, EMD having not yet developed the ability to produce their own bodywork. Like most boxcabs, they had control cabs at both ends, a feature that would only rarely be repeated in future North American locomotives, although it would become common elsewhere.
The two EMC demonstrators, numbered 511 and 512, were built in May 1935 to demonstrate the future of passenger Diesel power to potential customers. The boxy bodywork was not what EMC intended to sell, but it was an easy way to demonstrate the power units and hauling capacity, which would not be changed in the future E-units.
They were demonstrated both together and singly; the latter for shorter trains for local and less busy services, the former to replace larger steam locomotives on busy trains.
EMC #512, painted silver, served as Unit C of ATSF locomotive #1, and helped pull the first regular run of the streamlined, Budd-built Super Chief on May 18, 1937.
In 1938, having outlived their usefulness, the two demonstrators were scrapped. Trucks and some other components were re-used for the two NW4switchers built for the Missouri Pacific Railroad.
ATSF #1, formerly Unit A, after being rebuilt. The unit was also repainted in Santa Fe's classic Warbonnet colors.
Atchison, Topeka and Santa Fe Railway#1 was a two unit set built by St. Louis Car Company in late 1935 to haul the Santa Fe's new train, the Super Chief, for its first year of operation, from May 1936 until May of the following year. The Santa Fe had wanted the new, streamlined designs on the EMC drawing boards that would become the first E-units, but they would not be ready until 1937, so the railroad asked for two locomotives like the EMC demonstrators as proof of concept, letting the railroad gain some experience with Diesel operation before the E-units and the lightweight, streamlined train they would haul were ready. Because they were always run together, in a back-to-back configuration, Santa Fe employees nicknamed the two units the "One Spot Twins" and "Amos & Andy" (after the popular radio situation comedy). Both units shared a common road number, and the operating department considered them a single locomotive. The mechanical department referred to them as Unit A (lead unit) and Unit B (trailing unit). For a time, AT&SF leased EMC demonstrator #512, which became known as Unit C, while serving on the Super Chief.
The Santa Fe did ask for some cosmetic "dressing up" of the locomotives, since they would be hauling a prestige passenger train, and EMC obliged with a treatment by Sterling McDonald's GM styling department, which included large "eyebrow" air intakes at the front of the units and a striking paint scheme: Olive Green with Cobalt Blue and Sarasota Blue stripes separated by pinstripes of Crimson and Tuscan Red. This livery reduced the boxiness of the locomotives and gave them more of a look of speed. Later, as the Santa Fe took delivery of the stainless steel Super Chief passenger cars, the locomotives were repainted in silver.
The Santa Fe was an ideal railroad to be a Dieselization pioneer; its long desert runs in the Southwest made the provision of water supplies for steam locomotives problematic.
After the E1s replaced the proof-of-concept #1 in 1937, the Santa Fe began to modify the two locomotive units.
Unit A remained in passenger service. It was rebuilt as a single-ended locomotive in 1938 with a "bulldog" front end—a very high, raised cab above a snub rounded nose. The locomotive emerged in the Warbonnet paint scheme similar to the E1s. It retained road number 1. The lead truck was replaced with a drop-equalizer truck of unusual 1B configuration; the lead axle was unpowered, while the two rear axles were powered. Some time later, the trailing truck was replaced in similar fashion. Three-axle trucks rode better at speed and were lighter on the track, with a lower axle loading.
When Unit A was rebuilt in 1938, Unit B received the same modifications, along with road number 10, since it was now regarded as a separate locomotive. In 1941 #10 has its cab removed, and became a booster unit numbered 1A. In 1948, AT&SF rebuilt #1A into freight transfer locomotive #2611 running on EMD Blomberg B trucks.
Both #1 and #2611 went to EMD as trade-ins on E-8Bm locomotives in 1953.
9904 Pegasus, & 9905 Zephyrus were the original power for the second pair of Twin Zephyrs. Mechanically the same as the previous 1800 hp locomotives, they had stainless steel carbodies built by Budd. Their sloping front and streamlined styling continued the look of earlier Zephyr trainsets. Built to pull a specific carset, they were later pooled and continued in service until the mid 1950s.
Silver King (9906A), Silver Queen (9906B), Silver Knight (9907A), and Silver Princess (9907B), were the original power for the Denver Zephyr. Each 1800 hp cab (A) unit, repeats of the 9904-9905, was semipermantly attached to a 1200 hp booster(B) unit, for a total of 3,000 hp. Like 9904-9905, they were intended to pull a specific carset, but would later be pooled, and stayed in service until the mid 50s.