The Electro-Motive Division's (EMD) SD7 (“SD” stood for Special Duty)
was essentially the very same thing as a GP7 except that it sported a
C-C truck arrangement as opposed to the Geep’s B-B setup (meaning the SD
had six axles instead of four). For EMD, railroads at the time were
simply not interested in six-axle locomotives despite their added
benefits. Even similar designs
being offered by the American Locomotive Company (Alco), Baldwin and
Fairbanks Morse all sold poorly. Still, EMD would continue to offer
six-axle variants of popular General Purpose (GP) line until sales
finally began to take off with the SD40 of 1966. It is somewhat
surprising that despite less than 200 SD7s built a few still remain in
regular freight service.
Additionally, units officially preserved include Central of Georgia #201
at the Virginia Museum of Transportation and EMD's first SD7,
demonstrators #990 at the Illinois Railway Museum.
Electro-Motive SD7 demonstrator #990 seen here in February of 1952 at the La Grange plant. This unit, one of two the manufacturer built, went on to become Southern Pacific #5308 (the other, #991, became B&O #760).
The EMD SD7, which debuted in early 1952 (three years after the release of the GP7) was merely a step up from the GP7 model.
It 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 General Motors-built 16-cylinder 567B prime mover
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.
Burlington SD7 #324 appears to be tied down in West Eola, Illinois on August 27, 1969. Note that the CB&Q always included an attractive stencil of the model type just below the cab.
One additional advantage 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 which at 60 feet was about five feet
longer than the GP7 (to provide adequate space for the C-C trucks). The
SD7's two additional axles allowed it to produce far more tractive
effort than the GP7. Using GM's model D47 traction motor (the GP7
utilized the D27B) the SD7 could produce 75,000 pounds of continuous
tractive effort while the GP7 produce roughly half that amount, 40,000
pounds. Additionally, it offered a starting effort 90,800 pounds
compared to the GP7's 65,000 pounds. This also meant that the SD7 could
start a train much more quickly than its four-axle counterpart.
Great Northern SD7 #555 lays over in Minneapolis on the evening of June 9, 1964. The GN purchased 23 examples of this model in the early 1950s numbered 550-572.
In the early 1950s railroads still preferred four-axle diesel locomotives in main line freight service and as such, few early model SD designs
were constructed. The SD7, for instance, sold just 188 units, although
railroads like the Southern Pacific which had many routes with stiff
grades loved the model and used them in regular service for more than
four decades. Railroads that ultimately purchased the SD7 included the
Baltimore & Ohio (5, numbered 760–764), Bessemer & Lake Erie (8,
numbered 451–455, 801–803), Chicago & North Western (5, numbered
1660–1664), Burlington (37, numbered 300–324, 400–411), Milwaukee Road
(24, numbered 2200–2223), Colorado & Southern (10, numbered
810–819), Central of Georgia #201, Denver & Rio Grande Western (5,
numbered 5300–5304), Fort Worth & Denver (11, numbered 850–860),
Great Northern (23, numbered 560–572), Kennecott Copper #903,
Minneapolis & St. Louis (2, numbered 852, 952), Nevada Northern
#401, Pennsylvania Railroad (2, numbered 8588–8589), SP (43, numbered
5279–5293, 5308–5335), and Union Pacific (10, numbered 775–784).
The Southern Pacific was quite fond of the SD7 and later SD9 owning nearly 200 examples of both. What's more, many of these big six-axle units remained in service for more than four decades until the company disappeared into Union Pacific in 1996. Here, #1534 and a mate roll eastbound out of Roseville Yard in California during December of 1994.
EMD also constructed two demonstrators, #990 and #991 with the former
going to SP and the latter to the B&O. Despite their relative poor
sales numbers, several SD7s remain in service on shortlines, more than a
half-century since they first left EMD's shops in La Grange, Illinois
(a true testament to the reliability of EMD's first generation diesels).
Interestingly, SP kept their SD7s in service through the end with the
UP merger in 1996. Places you can still find SD7s include the Dakota
Southern, Tate & Lyle grain elevator in Mattoon, Illinois, Cargill's
grain elevator in Litchfield, Minnesota, Peavy grain elevator in
Jamestown, North Dakota, Portland & Western Railroad, and a few
others are stored away on sidings, some since forgotten.