The EMD SD24, "The First, Second-Generation, Six-Axle Power"

The Electro-Motive Division's (EMD) SD24 (“SD” stood for Special Duty) was the rarest of the early SD models although it is set apart from its earlier brethren (the SD7, SD9, and SD18) by being EMD’s beginning into second-generation power. While in just five years of production the first-generation, four-axle, GP7 and GP9 sold an astounding 3,436 units, nearly a 32% net increase in sales, their six-axle sisters, including the later SD24 were only marginally successful combining for roughly 700 units sold. Compare this with the GP7 and GP9, which are likely the best selling of all time, combining for over 6,000 units built when production ended on the GP9 in 1959.

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 SD24, which debuted in 1958 through 1963, while an extremely rare model and few were ever produced (179) it is included here because of its significance as EMD’s leap into second-generation power for six-axle locomotives. The unit was virtually identical to her GP30 (B-B trucks) sister in every other way except that she carried 2,400 hp instead of the GP30’s 2,250 hp.

For instance, the SD24 featured the very same 16-cylinder 567B, was equipped with the recently developed dynamic brake (a system for temporarily employing traction motors as generators and using the resulting electromotive force to slow the train), and featured an airtight hood that kept out dust, dirt, and other particles from reaching internal components (to cool these critical components the SD24 featured a single air intake for electrical cooling, with a pressurized cooling system).

The SD24's 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.

While the SD24 was not as successful as its early sisters it did pave the way for future second-generation six-axle models that would become legendary for their durability and reliability like the SD40, SD40-2, SD45, and others. Today, and while rare, a few of the early SD24 units survive throughout the country.

For more information on the SD24 series 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)!