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March 23, 2014 on the UP Kansas Sub in Kansas City, KS. A very rare appearance by UP 6936, the only remaining DDAX40 in service. It was picked up in Cheyenne, WY by train OWVBT 20. The "Centennial" locomotive is the largest diesel built by the Union Pacific Railroad.

Union Pacific 6936, the sole surviving operational EMD DDA-40X.

By KLWhitehead - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4812072

EMD DDA-40X Centennial Diesel Electric Locomotive
The DDA-40X is a 6,600 hp D-D diesel-electric built by the General Motors EMD division of La Grange, Illinois for the Union Pacific Railroad. Nicknamed "Centennial" and "Big Jack", the DD40AX uses two diesel engines (each 3,300 hp) and is the most powerful single-unit diesel locomotive ever built, although more recent locomotive designs such as the GE AC6000CW and EMD SD90MAC have come close. It is also the longest single-unit diesel locomotive ever built.

Description
In 1969 Union Pacific was retiring the later gas turbine-electric locomotives. Union Pacific had ordered EMD DD35s and DD35As to replace the turbines, and the DDA-40X was a further development. Forty seven were built between June 1969 and September 1971, except the first one delivered in April in time to participate in the celebrations of the centennial anniversary of the completion of the Transcontinental Railroad driving the "Gold Spike Limited" and arriving in Salt Lake City, Utah, on the morning of May 10, 1969. The units were numbered from 6900 to 6946, with 6936 still in service.

The DDA-40X is 98 feet long. The frames were fabricated by an outside contractor, the John Mohr Company of Chicago, since the frame length exceeded the abilities of EMD's plant. Using 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 designation stood for experimental, as the DDA-40X locomotives were used as the testbeds for technology that would go into future EMD products. The modular electronic control systems later used on EMD's Dash-2 line of locomotives were first used on the DDA-40X. The locomotive was the first to be able to load-test itself using its dynamic braking resistors as an electrical load so that external equipment was not required. The DDA-40X used the wide-nosed cab from the FP45 cowl units. This design was superficially similar to the Canadian comfort cab introduced by Canadian National soon afterwards in 1973, but it lacked the structural reinforcements introduced in the CN design that were carried over to future wide-nosed cabs.

As the DDA-40X program was a test, a number of experiments were conducted during the service life of these locomotives. One such test included fitting a few of the units with air raid sirens to warn track-side personnel when away from grade crossings, but the results were inconclusive. Another of the tests included successful modular electrical components. This made for easier diagnosis of electrical problems. These modifications were used in all future locomotive units built by EMD. All DDA-40X units included a new load test circuit, allowing units to load test without a track-side load test box. Gearing was 59:18, allowing 80 mph on passenger trains.

By 1974, averaging 22,000 miles a month, most DDA-40X units had run over 1,000,000 miles and needed more maintenance. In the early 1980s, this factor, along with other issues including the higher efficiency of newer locomotives such as the SD40-2 and the national recession led to all 45 remaining units into storage. In early 1984 as rail traffic rebounded, Union Pacific brought 25 DDA-40X units out of storage and rebuilt them to return to service; 15 non-serviceable units were retired in June that year. All DDA-40X units were retired by May 1985. Number 6936 however is still in service with UP, though mostly in excursion service.

Surviving examples
Thirteen examples survive, including one unit (6936) which is still in service with Union Pacific.

6900 - Kenefick Park, Omaha, Nebraska
6901 - Ross Park, Pocatello, Idaho
6911 - Mexico Museum of Technology, Mexico City
6913 - Museum of the American Railroad, Dallas, Texas
6915 - Southern California Chapter, Railway and Locomotive Historical Society, Fairplex, Pomona, California
6916 - Utah State Railroad Museum, Ogden, Utah
6922 - Cody Park, North Platte, Nebraska
6925 - Stored at Chamberlain, South Dakota, on Dakota Southern Railway
6930 - Illinois Railway Museum, Union, Illinois. Used as a control cab only, engines and motors are currently non-operational.
6936 - Still in service with Union Pacific Heritage Fleet
6938 - North Little Rock, Arkansas (Sits in front of Jenks Locomotive Facility)
6944 - Museum of Transportation, St. Louis, Missouri, sent to Altoona in July 2014 for cosmetic restoration, which was completed in May 2015 and is now back on display at Museum of Transportation, St. Louis, Missouri in June 2015.
6946 - Western Pacific Railroad Museum, Portola, California. The last DDA-40X built. This locomotive is fairly complete and on static display.

EMD DDA-40X Overview
Type and origin
Power type: Diesel-electric
Builder: General Motors Electro-Motive Division (EMD)
Build date: April 1969 – September 1971
Total produced: 47
Specifications
AAR wheel arrangement: D-D
Gauge: 4 ft 8 1⁄2 in (1,435 mm)
Wheelbase Between truck centers: 65 ft (19.81 m)
Truck wheelbase: 17 ft 1 1⁄2 in (5.22 m)
Length: 98 ft 5 in (30.00 m)
Width: 10 ft 4 in (3.15 m))
Height cab roof: 14 ft 11 3⁄8 in (4.56 m)
Height overall: 16 ft 4 in (4.98 m)
Locomotive weight: 521,980 lb (236,770 kg) or 260.99 tons (233.03 long tons; 236.77 t)
Fuel capacity: 8,230 gal (31,200 l; 6,850 imp gal)
Prime movers: 2 × EMD 645E3A
Engine type: V16 diesel
Aspiration: turbocharged
Cylinders: 16
Safety systems: Leslie Supertyfon model S5TRRO or S3LR horns, US&S Type 'EL' Cab Signals (#6936 equipped with US&S MicroCab ATC & CCS)
Performance figures
Power output: 6,600 hp (4,920 kW)
Tractive effort starting: 113,940 lbf (506.8 kN) @25%

Tractive effort continuous: 103,000 lbf (460 kN) @12 mph
Career
Operator: Union Pacific
Numbers: 6900 – 6946
Nicknames: "Centennial", "Big Jack"
Delivered: 6900-6924 April – December 1969
Delivered: 6925-6946 June 1970 – September 1971
Disposition: 1 in service, 46 withdrawn, (13 preserved, 34 scrapped)

See also:

GTEL

EMD Diesel-Electric Locomotives

EMD logo
Historic Rail - Shop Now!
Union Pacific 6936, the sole surviving operational EMD DDA-40X.

UP DDA-40X No. 6922 at Cody Park; North Platte, Nebraska.

Union Pacific's Centennial Locomotive, EMD DDA-40X 6,600 HP No. 6900. Union Pacific Railroad Photo.
UP wings herald

Union Pacific's Centennial Locomotive, EMD DDA-40X 6,600 HP No. 6900. This locomotive was built for the anniversary of the Golden Spike, and was the first of its class to be built and delivered in time for appearance at the Centennial Golden Spike ceremony, sometime between April and May 1969. Union Pacific Railroad Photo.