The SD90MAC, Tops in Horsepower

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 SD90MAC and it’s related models are, to date, the most powerful locomotive produced by EMD. The model hit the catalog in 1995 and EMD opted not to use its 16-cylinder 710G3B engine but the newly designed 16-cylinder H-engine (sometimes giving the model the name of SD90MAC-H), which allowed for the locomotive’s very powerful 6,000 hp. Aside from the prime mover the SD90MAC was essentially the same to the SD70 series and SD80MAC. One differing design was the SD90-43MAC, which is equipped with the 710-engine.

While the SD90-43MAC sold fairly well with Union Pacific, which purchased over 300 of them, the original SD90MAC did very poorly mainly because the engine was not properly tested and was prone to constant mechanical trouble. The original was also purchased by UP, along with Canadian Pacific and their total production ended at just 40 units. While over 400 units of the series have been produced (almost all of which are SD90-43MACs) the SD90MACs themselves were not well received by railroads because their lack of reliability and constant mechanical troubles (for instance, recently a set of SD90MACs went to work on the Appalachian & Ohio Railroad in central West Virginia only to be taken off the railroad due to mechanical problems).

After relative failures with the SD50 and SD60 EMD finally got back on track with the SD70, which was released in 1992, albeit at the cost of having already lost first place in the market to GE. The locomotive featured EMD’s 16-cylinder 710G3B engine and was rated at 4,000 horsepower although that was not its biggest selling point. Learning from the failures of the earlier models EMD upgraded its computer systems and microprocessors to be more easily maintained and introduced perhaps the biggest selling point of all; new, radial trucks.

Featuring the HTCR truck (high-traction, six-axle, radial) it could steer itself into oncoming curves instead of just following the rail which greatly reduced wear to both rail and truck/axle components. Not surprisingly railroads absolutely loved such a feature, which perhaps more than other reason made the SD70 series so successful.

While the original SD70 model was only marginally successful (it only sold over 100 units) later releases, such as the SD70M, SD70MAC, and SD70I were extremely successful. Essentially there is little difference between the designs in terms of overall mechanics and layout.

Across the industry the most successful SD70 has been the SD70MAC. The MAC carries the same wide-cab design as the SD70M except that it also features AC (alternating current) traction motors. Different from the traditional DC (direct current) traction motors AC is much more simple, reliable, and easier to maintain albeit it costs far more. This added cost, however, has not deterred railroads and over 1,000 have been built mostly for CSX and BNSF Railway (BNSF’s predecessor Burlington Northern also purchased a few hundred of them).

All in all EMD (today known as Electro-Motive Diesel as GM sold their locomotive division in 2005) has been very successful with the SD70 despite losing first place to GE and has now given their competitor stiff competition to its similar models the AC4400CW and new Evolution Series™.

In any event, be on the lookout for the SD90s, particularly on the UP as all of them are still in active service roaming around North America, including a few leasers who now operate a few of the units.

For more information on the SD90MAC series consider the book The American Diesel Locomotive from author Brian Solomon. The book studies everything from classic to newer diesel locomotive models. In all the book is a great resource and you shouldn’t be disappointed with it, not to mention that it is loaded with photos (many in color)! If you're interested in perhaps purchasing this book please visit The Railroad Diamond by clicking the tab in the menu to your left marked "TRD Store".