The EMD Six-Axle SD7, SD9 and SD18

EMD originally began as the Electro-Motive Corporation and has its beginnings in 1922 being based out of Cleveland, Ohio building inexpensive motorcars. With the purchase of the company in 1930 by General Motor, and more resources at its disposal the company began developing the precursor of the modern-day diesel-electric road unit (cabs and road-switchers), the streamlined trainset, which consisted of a powered car permanently attached to a few coaches (usually two or three cars). The most famous of these streamlined trainsets was the Burlington Zephyrs, a beautiful streamlined stainless steel creation (originally powered by Winton engines, which would also become a subsidiary of GM) that was extremely lightweight and fast. The original trainset, the Zephyr 9900, made headlines in 1934 when it completed a non-stop journey from Denver to Chicago in a little over 13 hours.

Because the power cars with these trainsets were permanently attached to the coaches EMC sought to design a locomotive free of this articulated setup but designed in such a manner that a railroad could still attach it to any passenger train and obtain the same, smooth streamlined look. This it did with the unveiling of the EA model in 1937, the first in a long line of passenger diesel locomotive designs that would come to be known as the E series.

Not surprisingly, soon after the EA unveiling the company decided to take things a step further and develop the first true diesel road unit, capable of pulling long freights in main line service. In 1939 it introduced the FT model (perhaps the key difference between the E and F series was that Fs rode on B-B trucks whereas Es rode on C-Cs and carried a noticeably longer carbody), the first in EMC’s F series and one of the most success diesel-electric designs of all time. The F series would go on to define American railroading for years and you can still see it in regular use today, over 60 years after it first debuted! The FT (which stood for Freight, Twenty-seven hundred horsepower) was a superb locomotive and although its 2,700 horsepower came from an A-B setup of a cab (A) and booster (B) units rated at 1,350 horsepower each, it was quickly loved by railroads for the efficiencies it held over steam such as its ruggedness and ease of maintenance.

The FT (which stood for Freight, Twenty-seven hundred horsepower) was a serious locomotive and although its 2,700 horsepower came from an A-B setup of cab (A) and booster (B) units rated at 1,350 horsepower each it would become embraced by the rail industry for the efficiencies it held over steam power as well as being rugged and easy to maintain.

Following the success of its cab units, EMD realized that there was a market to be made in the road-switchers, which at the time was mostly dominated by Alco with its RS series. Its first attempt at this type of locomotive, which gave the train crews both excellent vision all around the locomotive for switching and local service as well as enough horsepower to be used in main line operations, was the BL2. Although unsuccessful from a sales standpoint the BL2 was really a mere stepping-stone for its next model, the GP series (meaning General Purpose).

The SD7, which debuted in 1951 and manufactured through 1953, was EMD’s first six-axle (C-C) locomotive and virtually identical to her GP7 sister in every other way. For instance, the SD7 featured the very same 16-cylinder 567B engine and produced the same 1,500 hp.

Its primary purpose was that the extra two axles produced more traction (which allowed the locomotive to handle stiffer grades), allowed for better weight distribution (which was a big plus on light rail and bridges unable to support heavy loads, found on many branch lines) and its Flexicoil trucks allowed for ease of maintenance on its center traction motor. One last standpoint of the SD7 was that while competitors offered six-axle models based from the same frame of a four-axle, EMD designed its very own frame for the SDs.

The later SD9 and SD18 models, built between 1954 and 1963, were virtually the same in every other way except that they featured a bit more horsepower.

Of all the models the SD9 was by far the most successful selling around 471 units (many to the Southern Pacific which had a lot of rugged territory, especially in the Pacific Northwest) and because of this they are the most prolific of the three models still roaming around the country today, although a few SD7s and SD18s are still around as well.

For more information on the early "Special Duty" locomotive models consider one (or both) of the books below. Mike Schafer’s Vintage Diesel Locomotives looks at virtually all of the classic builders and models from Alco PAs to early EMD Geeps. If you’re interested in classic EMDs, or diesels in general, this book gives an excellent general history of both.

Also, Brian Solomon’s The American Diesel Locomotive also studies many of the same diesel models and builders as Mr. Schafer’s publication although at nearly twice the size the book goes into a bit more detail than Vintage Diesel Locomotives and also covers newer models. All in all, though, both books are a great read and you shouldn’t be disappointed with them, not to mention that they are loaded with photos (many in color)!