Union Pacific Centennials (DDA40X)

Union Pacific Centennial

Surviving Centennials Centennial Specifications Centennial Pages on the Web Centennial References

The Union Pacific Railroad earned a reputation for using very large and powerful steam locomotives and in 1941 began using the largest of them all, the Big Boy, to carry freight over the Rocky Mountains.

In the diesel era, the tradition continued with the use of the most powerful locomotives available. In 1969 UP purchased the very largest and most powerful diesel ever built, the DDA40X, with 6600 HP provided by two diesel engines (prime movers) mounted on one frame,

These 270 ton, 98 foot long behemoths were built by the Electro-Motive Division of General Motors Corp. (EMD) and were named "Centennial" in honor of the 100th anniversary of the "Driving of the Golden Spike Ceremony" (May 10, 1869). In all, 47 units were purchased (numbered 6900-6946) beginning in May of 1969 with deliveries continuing through 1971. They were numbered using the "6900" series to mark the '69 dates.

The "Centennials" were designed for high-speed freight and by 1980 had successfully run up an average of 2 million miles a piece. With the decline of freight movement in 1980 the fleet of DDA40Xs were taken out of service and put in desert storage at Yermo, CA. Then in March of 1984 an economic recovery brought a demand for more motive power and 25 were returned to service. However, high maintenance costs caused the retirement of most of them by the close of 1986.

Today 13 of these double diesel locomotives survive and can be visited as shown below. One unit #6936 has recently had a major overhaul and is still owned and regularly operated by the UPRR.

Surviving Centennials

6900: Lauritzen Gardens/Kenefick Park, Omaha, NE

Displayed with Big Boy 4023. Built 4/69, Donated 6/88. Units 6900, 01 & 02 were delivered with a 7000 HP rating using 20% fuel injectors. They were returned to 6600 HP when 10% injectors were installed in 1970.

6901: Ross Park, Pocatello, ID

Displayed with UP 2-8-2 #2005. Built 8/69, Donated 4/86.

6904: National Railway Equipment Co., Mt. Vernon, IL

Built 7/69, Sold 8/88, Scrapped 2/98

6911: Technology Museum, Mexico City, Mexico

Built 9/69, Donated 3/85

6913: Museum of the American Railroad, Frisco, TX

Displayed with Big Boy 4018. Built in 1969 at a cost of $551,168 ($2,557,960 in 1998 dollars) it ran 2,000,000 miles before it was retired in 1986. Built 10/69, Donated 3/86.

6915: Rail Giants Train Museum, Pomona, CA

Built 10/69, Donated 4/86.


6916: Ogden Union Station Museum, Ogden, UT

Built 11/69, Donated 4/86.

6922: Cody Park Railroad Museum, North Platte, NE

Displayed beside Challenger 3977. Built 12/69, Retired 4/85, Donated 8/85.

6925: Dakota Southern Railway, Chamberlain, SD

Owned by Dakota Southern Railway and parked in Chamberlain, SD yard. Traction motors, rads and prime movers have been removed. Built 6/70, Sold 5/87.

6930: Illinois Railway Museum, Union, IL

Built 8/70, Donated 6/85.

6936: UP's Steam Shop, Cheyenne, WY

Owned by the Union Pacific Railroad. Built 1/71, 6936 is the only "Centennial" still in working order and is used for internal company special trains.

6938: UP's Jenks Locomotive Shop, North Little Rock, AR

Built 6/71, Retired 1/90.
  • 6938 (2019 Little Rock, AR photo by Wes Barris)
  • 6938 (2019 Little Rock, AR photo by Wes Barris)
  • 6938 (2019 Little Rock, AR photo by Wes Barris)
  • photo

6944: National Museum of Transportation, St. Louis, MO

Displayed with Big Boy 4006. Built 8/71, Donated 8/84.

6946: Western Pacific Railroad Museum, Portola, CA

Built 9/71, Donated 8/84.
  • 6946 (1986 photo by Chris Skow)
  • 6946 (1987 photo by Randy Rebernick)
  • 6946

EMD, Union Pacific and the "DDs"

When diesels took over on the Union Pacific, the power of steam was not easily matched with some trains requiring as many as eight diesel locomotives to move freight over the continental divide. UP management wanted a better way of providing motive power and EMD responded with a 5000 HP cabless booster unit. This unit was designated "DD35" and was built with off-the-shelf components and was essentially two standard GP35's on a common frame and ran on two four-axle trucks.

Many considered the four-axle truck arrangement to be very hard on tracks, but built as a cabless booster unit it would never be in the lead, and the effect on the tracks would be acceptable. EMD built two demo units for UP (road numbers 72B & 73B) and delivered them in September, 1963.

In April, 1964, three demo units were delivered to Southern Pacific (road numbers 8400-8402 which later became 9500-9502 and still later became 9900-9902). SP did not respond with purchases of additional units and only used the three booster units they bought sparingly until they were retired in the 1970's.

Meanwhile, Union Pacific, which had and still has excellent tracks, ordered 25 of the DD35 booster units (road numbers 74B through 98B) (Railfan.net Usenet ABPR Picture Archives) and requested that a cab be added to all future orders. UP later bought 15 of the units with cabs which were designated "DD35A" (road numbers 70 through 84).

The moderate success of the DD35A and the introduction of the GP40 led very quickly to the development of the DDA40X which was basically the DD35A with increased HP. Union Pacific bought 47 of these units. The cost effectiveness of the "DD's" was never fully disclosed, however they were shorter than two GP's and obviously saved on the cost of cabs. But the greatest savings would have been from the removal of a great deal of the troublesome electrical control equipment that was required to operate multiple units.

Continued development of more reliable and easier to maintain modular electrical systems along with the introduction of the 20 cylinder, 3600 HP "SD45-2" allowed for convenient selection of available HP and reduced the appeal of double diesel locomotives.

Based on the HP rating the DDA40X is the most powerful diesel locomotive ever built and coupled with the special introduction and the romance in the name "Centennial" it has become an icon for diesel rail fans.

Centennial Specifications

Wheel Arrangement2 sets of quadruple powered axles
Length98'-5" (over couplers)
Height17'-4" (track to over horn)
Weight545,432 lbs (with full load of fuel and fluids)
Engines(2) 16-645E3A
Traction Motors(8) D-77X
Total HP6,600HP
Tractive Effort136,000 lbs at start and 19,800 lbs at 90 mph
Wheels40 inch diameter
Gear Ratio59/18
Top Speed80 - 90 mph
Fuel Capacity8,200 gallons
Lube Oil305 gallons per engine
Cooling Water300 gallons per engine

Centennial Web Pages

Centennial Reference

  • Giants of the West by George Cockle (Overland Books / George Cockle Associates, Phone 402-553-4744)
  • Centennials in Action by George Cockle (Overland Books / George Cockle Associates, Phone 402-553-4744)
  • Last of the Giants by Jerry T. Moyers and Jim Boyd in the Fall 1975 issue of Railfan


Much thanks to Richard Duley who has helped to provide a great deal of the information found on this page.
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Wes Barris