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UP DDA40X #6922 at Cody Park; North Platte, Nebraska.
Type and origin
Power type Diesel-electric
Builder General Motors Electro-Motive Division (EMD)
Build date April 1969 – September 1971
Total produced 47
 • AAR D-D
Gauge 4 ft 8 12 in (1,435 mm)
Wheelbase Between truck centers: 65 ft (19.81 m)
Truck wheelbase:
20 ft 1 12 in (6.13 m)
Length 98 ft 5 in (30.00 m)
Width 10 ft 4 in (3.15 m))
Height cab roof: 14 ft 11 38 in (4.56 m)
overall: 16 ft 4 in (4.98 m)
Loco weight 545,000 lb (247,000 kg)
or 260.99 short tons (233.03 long tons; 236.77 t)
Fuel capacity 8,280 US gal (31,300 l; 6,890 imp gal)
Prime mover 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% continuous: 103,000 lbf (460 kN) @12 mph
Operators Union Pacific
Numbers 6900 – 6946
Nicknames "Centennial", "Big Jack"
Delivered 6900-6924 April – December 1969
6925-6946 June 1970 – September 1971
Disposition 1 in service, 46 withdrawn, (13 preserved, 34 scrapped)

The DDA40X is a 6,600 hp (4.92 MW) 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"[citation needed], the DDA40X uses two diesel engines (each 3,300 hp (2.46 MW)) 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 the second most powerful locomotive ever built, losing to the Union Pacific GTEL. It is also the longest single-unit diesel locomotive ever built.[1]


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 DDA40X 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 DDA40X is 98 ft (30 m) 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 DDA40X 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 DDA40X and the 4200 HP SD45X. The locomotives were 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 DDA40X 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.[2]

As the DDA40X 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 built by EMD. All DDA40X 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 freight trains.

By 1974, averaging 22,000 miles a month, most DDA40X 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 DDA40X units out of storage and rebuilt them to return to service; 15 unserviceable units were retired in June that year. All DD40AX units were retired by May 1985.[1][3] Number 6936 however is still in service with UP, though mostly in excursion service.

Surviving examples[edit]

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

Union Pacific #6936 is the only DDA40X still in service with Union Pacific Railroad, though it is mostly used to haul excursions.
The engineer's control stand.


  1. ^ a b Don Strack. "Utah Rails DDA40X page". Utahrails.net. Retrieved 28 January 2015. 
  2. ^ CN Locomotive Cab Layout GR-20c, Commission of Inquiry Hinton Train Collision, December 1986
  3. ^ "Union Pacific Railroad Roster of Diesel and Turbine Locomotives, 1934-2009". Utahrails.net. Retrieved 2 September 2015. 
  • 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]