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Union Pacific 6922 on display at Cody Park, North Platte, Nebraska
Type and origin
Power typeDiesel-electric
BuilderGeneral Motors Electro-Motive Division (EMD)
Build dateApril 1969 – September 1971
Total produced47
 • UICDo'Do'
Gauge4 ft 8+12 in (1,435 mm)
WheelbaseBetween truck centers: 65 ft (19.81 m)
Truck wheelbase:
20 ft 1+12 in (6.13 m)
Length98 ft 5 in (30.00 m)
Width10 ft 4 in (3.15 m)
Heightcab roof: 14 ft 11+38 in (4.56 m)
overall: 16 ft 4 in (4.98 m)
Loco weight545,000 lb (247,000 kg)
or 260.99 short tons (233.03 long tons; 236.77 t)
Fuel capacity8,280 US gal (31,300 L; 6,890 imp gal)
Prime mover2 × EMD 16-645
Engine typeV16 diesel
Safety systemsLeslie Supertyfon model S5TRRO or S3LR horns, US&S Type 'EL' Cab Signals (6936 equipped with US&S MicroCab ATC & CCS)
Performance figures
Maximum speed80–90 mph (129–145 km/h)
Power output6,600 hp (4,920 kW)
Tractive effortstarting: 113,940 lbf (506.8 kN) @25% continuous: 103,000 lbf (460 kN) @12 mph
OperatorsUnion Pacific
Numbers6900 – 6946
Delivered6900-6924 April – December 1969
6925-6946 June 1970 – September 1971
Disposition11 in museums, 1 operational, 1 pending disposition, 34 scrapped

The EMD DDA40X is a 6,600 hp (4,943 kW) D-D locomotive, built by EMD from 1969 to 1971 exclusively for the Union Pacific Railroad.[1] It is the most powerful diesel-electric locomotive model ever built on a single frame, having two 16-645E3A diesel prime movers.[2] Union Pacific has marked DD40X on the cab exteriors, while EMD literature inconsistently refers to this model as either DD-40X or DDA40X.[3]

UP's DDA40X locomotives were the culmination of the company's experiments with extremely powerful locomotives that began with its gas turbine-electric locomotives and DD35s.[4] For manufacturer EMD, the construction of the world's most powerful single frame locomotive was a sign of the company's dominance of the North American diesel locomotive market, with only GE Transportation an equal competitor. The DDA40X also pioneered a number of new technologies that would go on to be incorporated in future EMD designs.

Ultimately, UP did not continue with exceptionally powerful locomotives like the DDA40X built on single frames, instead moving towards distributed power using smaller locomotives closer to 4,000 hp. All of DDA40X locomotives were retired between 1984 and 1986; several of them survive. Union Pacific 6936 operated as a member of the Union Pacific Heritage Fleet until 2022, when it was announced it would be donated to the Railroading Heritage of Midwest America museum.[1][5]


In 1969, Union Pacific was retiring the last of their gas turbine-electric locomotives. Beginning in 1963, Union Pacific had ordered EMD DD35s and DD35As to replace the turbines, and the DDA40X was a further development from this design.[4]

The first DDA40X, UP 6900, was delivered in April 1969, in time to participate in the celebrations of the centennial anniversary of the completion of the first transcontinental railroad driving the "Gold Spike Limited"; it arrived in Salt Lake City, Utah, on the morning of May 10, 1969. In honor of this, the class was nicknamed Centennials[4] and the choice of locomotive numbers in the 6900s was made for the same reason.[1] Forty-six more were built between June 1969 and September 1971, numbered from 6901 to 6946.

DDA40X 6943 next to GE C30-7 2400, demonstrating its comparative length on the Keddie Wye of the Feather River Route (1984)

The DDA40X is 98 ft (30 m) long. The frames were fabricated by the John Mohr Company of Chicago, because they were too large for EMD's factory. The use of more than one prime mover in a single locomotive was not new; the E-series were popular dual-engine locomotives, and Baldwin had produced (but not sold) a locomotive with four diesel engines.

The "X" in the model number stood for experimental, as DDA40X Centennials were testbeds for technology that would go into future EMD products. UP frequently used these locomotives to haul heavy freights. Each unit successfully ran about two million miles (3,200,000 km) before they were retired from revenue service in 1985. The modular electronic control systems, later used in EMD Dash-2 models, were first used on the DDA40X and the 4,200 hp SD45X. All DDA40X units included a new load test circuit, whose dynamic braking resistors allowed units to load test without a track-side load test box. Gearing was 59:18, allowing 80 mph (130 km/h) on freight trains.

The DDA40X has a wide nose akin to those on the F45 and FP45 cowl units. These cabs were superficially similar to the Canadian comfort cab introduced by Canadian National in 1973, though without the structural reinforcements of the Canadian design.[6]

Other experiments were conducted during the service life of these locomotives. A few of the units were fitted with Federal Signal Thunderbolt air raid sirens to warn track-side personnel when away from grade crossings, but the results were inconclusive.[7][8][better source needed] Another test included the modular electrical components, which was successful. This made for easier diagnosis of electrical problems. These modifications were used in all future locomotives built by EMD.

Despite their excellent performance and relatively good efficiency, these units were costly to maintain, which ultimately prompted Union Pacific to begin placing them into storage in the early 1980s. But in early 1984, as rail traffic rebounded, Union Pacific brought 25 units out of storage and rebuilt them to return to service.[9][10] All locomotives were finally retired by 1986. Eleven DDA40X units are preserved by various museums, while another unit survives as a source of spare parts for other locomotives. UP 6936, the sole operating unit, was owned by Union Pacific and was used in excursion service until 2016. In 2022, this locomotive was donated by Union Pacific to the Railroading Heritage of Midwest America, which plans to return the locomotive to operation.[11] 6936 was operated by the museum for the first time in August 2023.[12]

Surviving examples[edit]

Union Pacific 6936 pulling a business train in Watseka, Illinois

Thirteen DDA40Xs survive today. The following list details the surviving locomotives and their current owners.

See also[edit]


  1. ^ a b c "Centennial DD40X". UP.com. Retrieved July 30, 2021.
  2. ^ Solomon, Brian (June 15, 2016). "EMD DDA40X". The field guide to trains : locomotives and rolling stock. Minneapolis, Minnesota. p. 189. ISBN 9780760349977. OCLC 928614280.{{cite book}}: CS1 maint: location missing publisher (link)
  3. ^ "Diesel-Electric Locomotives".
  4. ^ a b c Schafer, Mike (1998). Vintage diesel locomotives. Osceola, WI: Motorbooks International. p. 40. ISBN 0-7603-0507-2. OCLC 38738930.
  5. ^ Franz, Justin (April 28, 2022). "UP Donates Two Steam Locomotives to Non-Profit; 'Challenger' to Run Again". railfan.com. White River Productions. Retrieved April 28, 2022.
  6. ^ CN Locomotive Cab Layout GR-20c, Commission of Inquiry Hinton Train Collision, December 1986
  7. ^ Edmund (September 4, 2018), UP_6918_TB-1003, retrieved December 2, 2022
  8. ^ Edmund (December 15, 2008), UP_DDA40X_TB-1003, retrieved December 2, 2022
  9. ^ Strack, Don. "Union Pacific DDA40X Centennial Locomotives". Retrieved April 7, 2011.
  10. ^ "EMD DDA40X Centennial". Trains. Retrieved December 17, 2022.
  11. ^ Glischinski, Steve (April 28, 2022). "Railroading Heritage of Midwest America, Union Pacific agree to donation of Challenger, other locomotives, cars". Trains. Kalmbach Publishing. Archived from the original on April 28, 2022. Retrieved April 28, 2022.
  12. ^ Franz, Justin (September 6, 2023). "Union Pacific 'Centennial' Restored to Service in Illinois". Railfan & Railroad Magazine. Retrieved September 6, 2023.
  13. ^ Franz, Justin (October 24, 2022). "Struggling Nebraska Museum Sells UP Centennial". Railfan & Railroad Magazine. Retrieved November 15, 2022.
  • Union Pacific Railroad Locomotive Department (1979). Locomotive Diagram Book. Union Pacific Railroad Company.
  • Union Pacific Railroad Locomotive Department (1994). Locomotive Diagram Book. Union Pacific Railroad Company.
  • Hayden, Bob, ed. (1980). Model Railroader Cyclopedia-Volume 2: Diesel Locomotives. Kalmbach Books. ISBN 0-89024-547-9.

External links[edit]