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EMD "SD7" Locomotives

Last revised: January 11, 2024

By: Adam Burns

When Electro-Motive quietly unveiled the six-axle SD7 in 1952, company officials could have never imagined the new "Special Duty" line would one day witness unprecedented success.

Prior to its introduction, EMD had no interest in a specialized locomotive and instead remained focused on mass produced four-axle variants. At the time this included the streamlined "F" series although the builder had recently (1949) launched its own road-switcher line, the GP7.

The SD7 was the Geep's larger cousin, sporting a C-C truck arrangement that offered greater tractive effort on a longer frame but was otherwise identical.  At the time, there was little interest in such a design; comparable models sold by Alco and Baldwin also found little interest.

A total of 188 SD7s were produced during a very short production run from April, 1952 to November, 1953.  Interestingly, in a nod to the quality of EMD products, the model's largest buyer - Southern Pacific - rebuilt its entire fleet, which remained in service through the 1990s. 

Photos

Burlington Northern (Colorado & Southern) SD7 #6077 (built as #817) and GP38-2 #2154 at North Yard in Saginaw, Texas, circa 1983. Mike Bledsoe photo. American-Rails.com collection.

History

The six-motored locomotive concept was not new when EMD introduced the SD7.  As Brian Solomon points out in his book, "EMD Locomotives," General Electric had offered a straight-electric in this configuration as early as 1930 when it produced 42 Class R-2 locomotives for New York Central for its West Side freight operations in New York City (Manhattan).

In addition, Alco had first offered a six-motored RS1 in 1942 for the U.S. Army while Baldwin first cataloged its own six-axle version as early as 1946, the A1A-A1A trucked DRS-6-4-1500.  Two years later it unveiled the six-motored DRS-6-6-1500.

Internally, the SD7 was identical to its four-axle counterpart, equipped with a 1,500 horsepower, 16-cylinder model 567B prime mover.

However, it was also unique via a custom designed frame; by contrast, Alco and Baldwin had utilized the same basic frame in developing both their four, and six-axle models.  While internally the same, the SD7 was outwardly unmistakable from the GP7. 

At a length of 60 feet, 8 ½ inches over couplers, the model was more than five feet longer than its four-axle counterpart.  A single locomotive could haul a 5,500-ton train on level track, climb grades as high as 5%, and negotiate curves of 23 degrees.

The SD7 also featured another unique trait, EMD's own Flexicoil truck. This lightweight, high adhesion design offered a surprisingly comfortable ride and was easy to maintain.  It provided mechanics straightforward access to the center-axle, which was difficult to reach in Alco's early six-axle truck.

75991907762787289019772980.jpgBurlington Northern SD7 #6088, sub-lettered for the Fort Worth & Denver, is seen here in Fort Worth, Texas, circa 1983. Mike Bledsoe photo. American-Rails.com collection.

Reception

In the early 1950s, railroads still preferred four-axle locomotives in main line freight service.  As a result, reception to EMD's early six-axle variants was cool.

Nevertheless, there was some interest, especially by Southern Pacific, which tested out the builder's bright red demonstrator, #990, and ultimately purchased this unit.  In fact, SP wound up with the largest fleet, 43 of the 188 examples produced.

Data Sheet and Specifications

Entered Production4/1952 (Milwaukee Road #2200)
Years Produced4/1952 - 11/1953
Engine567B
Engine BuilderGM
Horsepower1500
RPM800
Cylinders16
Length60' 8 ½"
Height (Top Of Rail To Top Of Cab)15' 0"
Width10' 8"
Weight300,000-360,000 Lbs
Fuel Capacity1200 Gallons
Air CompressorGardner-Denver
Air Compressor ModelWBO
Air Brake ManufacturerWestinghouse
Air Brake Schedule6BL
TrucksC-C
Truck TypeFlexicoil
Truck Wheelbase13' 7"
Wheel Size40"
Traction MotorsD27C (6), GM
Primary GeneratorD12B, GM
Auxiliary GeneratorDelco
AlternatorD14
MU (Multiple-Unit)Yes
Dynamic BrakesYes
Gear Ratio65:12, 62:15, 61:16, 60:17, 59:18, 58:19
Tractive Effort (Starting)90,800 Lbs at 25%
Tractive Effort (Continuous)75,000 Lbs at 9.3 mph
Top Speed55 mph (65:12), 65 mph (62:15), 71 mph (61:16), 77 mph (60:17), 83 mph (59:18), 89 mph (58:19)

Production Roster

Total Built = 188

Owner Road Number(s) Serial Number(s) Order Number Completion Date
Milwaukee Road 2200-2208 15612-15620 5139 4/1952-6/1952
Milwaukee Road 2209-2211 15621-15623 5201 6/1952, 8/1952
Electro-Motive (Demonstrator) 990 (became Southern Pacific #5308) 15624 7512 10/1952
Great Northern 550-564 16099-16113 5146 5/1952-10/1952
Nevada Northern 401 16824 6397 8/1952
Milwaukee Road 2212-2214 16941-16943 5144 6/1952
Fort Worth & Denver City (Burlington) 850-857 17121-17128 6468 3/1953
Colorado & Southern (Burlington) 810-817 17129-17136 6274 1/1953-2/1953
Bessemer & Lake Erie 801-803 17138-17140 6436 10/1952
Southern Pacific 5279-5293 17144-17156 5157 11/1952-1/1953
Colorado & Southern (Burlington) 818-819 17260-17261 6274 2/1953
Chicago, Burlington & Quincy 400-411 17396-17407 5140 11/1952-12/1952
Minneapolis & St Louis 852, 952 17408-17409 5181 12/1952
Electro-Motive (Demonstrator) 991 (became Baltimore & Ohio #760) 17410 6457 3/1952
Kennecott Copper Corporation 903 17411 6456 12/1952
Great Northern 565-572 17896-17903 5182 3/1953-4/1953
Bessemer & Lake Erie 451-455 17904-17908 5212 4/1953
Fort Worth & Denver City (Burlington) 858-860 17921-17923 5249 10/1953
Denver & Rio Grande Western 5300-5304 18124-18128 5232 5/1953
Chicago & North Western 1661-1664 18223-18227 5185 5/1953
Chicago & North Western 1660 18227 5276 6/1953
Union Pacific 775-781 18284-18290 5244 6/1953
Union Pacific 783-784 18291-18292 5277 6/1953
Union Pacific 782 18293 5278 6/1953
Milwaukee Road 2215-2219 18301-18305 5274 10/1953
Milwaukee Road 2220-2223 18306-18309 6516 10/1953
Central of Georgia 201 18310 6512 5/1953
Chicago, Burlington & Quincy 300-324 18320-18344 5242 5/1953-10/1953
Southern Pacific 5316-5335 18381-18400 5188 8/1953-9/1953
Southern Pacific 5309-5313 18401-18405 5282 7/1953
Southern Pacific 5314 18406 5283 7/1953
Southern Pacific 5315 18407 5286 7/1953
Pennsylvania 8588-8589 18669-18670 5300 10/1953
Baltimore & Ohio 761-764 18683-18686 5303 11/1953

An Electro-Motive builder's photo showcasing new Great Northern SD7 #558 in the summer of 1952. Author's collection.

Demonstrators

In designing the SD7, EMD intended it to be used in light branch service.  The truck itself was lightweight and its additional axle could more evenly distribute the locomotive's weight over a greater distance, a handy trait in territory where rail was generally light.

Interestingly, the Flexicoil proved so successful it remained EMD's standard through the early 645 series which included the SD38, SD40, and SD45.  It was finally replaced with the "Dash 2" line, which carried the new HT-C truck.

EMD's second demonstrator, #991, was purchased by the Baltimore & Ohio, which went on to roster five examples, #760-765.  Other buyers included the Milwaukee Road, Great Northern, Nevada Northern, Burlington (and subsidiaries), Bessemer & Lake Erie, Minneapolis & St Louis, Kennecott Copper Corporation, Rio Grande, Chicago & North Western, Union Pacific, Central of Georgia, and Pennsylvania.

Interestingly, SP kept their SD7s in service - following an extensive rebuild program into SD7Rs, through the end when the railroad merged with Union Pacific in 1996.

Sources

  • Foster, Gerald. A Field Guide To Trains. New York: Houghton Mifflin, 1996.
  • Hayden, Bob. Diesel Locomotives: Cyclopedia, Volume 2 (Model Railroader). Milwaukee: Kalmbach Publishing Company, 1980.
  • Marre, Louis A. Diesel Locomotives: The First 50 Years, A Guide To Diesels Built Before 1972.  Milwaukee: Kalmbach Publishing Company, 1995.
  • Pinkepank, Jerry A. Diesel Spotter's Guide.  Milwaukee: Kalmbach Publishing Company, 1967.
  • Schafer, Mike. Vintage Diesel Locomotives. Osceola: MBI Publishing, 1998.
  • Solomon, Brian.  EMD Locomotives.  Minneapolis: MBI Publishing Company, 2006.

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