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Athearn HO scale EMD DDA40X diesel locomotive

Read this review from Model Railroader
Athearn HO scale Electro-Motive Division DDA40X diese locomotive
Athearn HO scale Electro-Motive Division DDA40X diesel locomotive
When I took the Athearn Genesis series DDA40X out of the box, I was impressed by its sheer size and heft. Then I put this HO scale model of the world’s largest diesel-electric locomotive on the track and was soon in awe of its sound system and pulling power.

The dual-motored model has two SoundTraxx decoders and front and rear speakers. In addition, the model’s lines and details match its famous prototype.

The prototype

Between 1969 and 1971, General Motors’ Electro-Motive Division delivered 47 DDA40X locomotives to the Union Pacific RR. Since UP received the first DDA40X a few weeks before the 100th anniversary of the completion of the transcontinental railroad, the locomotives were nicknamed “Centennials.”
The locomotive rode on two “D,” or four-axle, trucks. The designation A stood for cab unit, while “X” indicated the locomotive’s experimental status. The “40” stood for the 16-cylinder 645E3 engine that was also used in EMD’s GP40 and SD40.

Each DDA40X was equipped with two of these 3,300 hp prime movers for a total 6,600 hp, making the Centennials the most powerful diesel-electric locomotives ever built. They were also geared to run at a top speed of 90 mph.

The UP began retiring the locomotives in 1984. However, 13 Centennials have been preserved.

The model

The model’s dimensions match drawings in Model Railroader Cyclopedia: Vol. 2, Diesel Locomotives (Kalmbach Publishing Co., out of print).

The plastic body shell features an exceptional level of detail. See-through grills are mounted over separate fans on the roof. Other individual details include sunshades and the firecracker radio antenna on the cab and brake cylinders on the truck sideframes.
The passageway between the engine compartments is prototypical
The passageway between the engine compartments is prototypical.
As on the prototype, the model has a walkway between the two engine compartments. Another prototypical touch is the separate piping modeled inside this passageway.
The cab interior is detailed with a brake stand, cab speedometer, and other separate detail parts. There are also seats for the engineer, fireman, and head-end brakeman. No figures are included, so the modeler will have to add his own.

The models are available in multiple road numbers with the as-delivered paint scheme featuring UNION PACIFIC RAILROAD in the shield herald and the later paint scheme with only UNION PACIFIC in the shield.The paint and lettering on our review samples matches prototype photos; even the tiny FIRE EXTINGUISHER INSIDE stencils are legible under magnification.
The model features two flywheel motors
The model features two flywheel-equipped motors. The direct-current version shown above requires two decoders for conversion to Digital Command Control.
Getting under the hood

Eight screws hold the body shell to the die-cast metal frame. First, I removed the front and rear couplers. Then I removed the screws between the fuel tank and the trucks. Two more screws attach the plastic fuel tank to the die-cast metal frame. Once I removed those screws I easily lifted off the fuel tank. This reveals two additional screws in the center of the frame that I removed. I could then easily lift off the body shell. Note: Don’t remove the eight smaller screws that are visible on the bottom of the frame. These hold the motors in place.
The two motors sit in the center of the frame. The front motor, which powers the front truck, has dual brass flywheels, while the rear motor, which powers the rear truck, has a single flywheel.

A printed-circuit board is mounted on top of each motor. The DCC version features two SoundTraxx sound decoders. There are also two vertically mounted 28mm speakers: one behind the cab and the another at the rear of the frame.

The DC version features two eight-pin and nine-pin DCC sockets. Frame-mounted speaker enclosures are also included. To convert the DC version to DCC, two decoders are required.

In addition to front and rear  headlights, the model has a flashing beacon on the cab roof. Instructions for replacing these light bulbs are included with the model. Our review samples (nos. 6904 and 6925) don’t have ditch lights, which would have been found on Centennials at the end of their careers in the 1980s.
Athearn HO scale DDA40X

The model ran smoothly on our DC and DCC test tracks. Like other sound-equipped models, the DDA40X requires a lot of voltage – 7.75V – to start moving on a DC layout. I appreciated that the lights and sounds came on earlier at 6V.

As the speed charts on the right show, the model has a prototypical speed range. The model’s top speed was slightly higher during DC speed tests.

After setting the model’s decoders from 28 speed steps to 128 steps, its low speed in DCC was only 1 scale mph.

Like the prototype, the DDA40X requires broad curves. I ran the model through a 28" radius curve and no. 6 turnouts without any problems.

The model doesn’t have traction tires, yet still produced 5 ounces of drawbar pull, which is enough to haul 70 freight cars on straight and level track. The Centennial is also quite a hill climber. The model hauled a 23-car train up a 3 percent grade at 50 scale mph.


According to Athearn, the HO scale locomotive features sounds recorded from prototype DDA40X no. 6936. In both DC and DCC the dual decoders and speakers accurately replicate the sound of two separate diesel engines. Like the prototype, the separate engine rpm went in and out of phase.

The horn sounds like prototype videos that I’ve seen. The decoder features two other Leslie air horn options.

On a DC layout, the sounds are automatic, unless you have a sound controller like a Model Rectifier Corp. Tech 6. These automatic sounds include a bell that turns on between 8.5 and 9V and a grade crossing signal that sounds when the direction switch is flipped. The automatic sounds can be changed or turned off, but this programming requires a Tech 6 or a DCC system.

In DCC, user-triggered functions include the horn, bell, and dynamic brake fans. Engine sounds can be fine-tuned, including optional manual notching. That way the engine rpm can be increased or decreased independently of the locomotive’s speed.

A list of programmable configuration variables (CVs) is included. The CVs control many of the lighting, sound, and performance features. A more extensive manual with tips for programming the decoder is also available on the Athearn website or at

The Athearn DDA40X is the first HO scale model I’ve heard that accurately replicates the sounds of a dual-motored locomotive. The model’s pulling power is also worthy of its massive prototype.
Price: $369.98 (direct-current), $499.98 (DCC sound)

Athearn Trains
2883 E. Spring St., Ste. 100
Long Beach, CA 90806

Era: 1969 to 1985
  • All-wheel drive and electrical pickup
  • Blackened metal RP-25 contour wheels in gauge
  • DCC quick plugs on direct-
  • current version (Note: Two decoders required for DCC operation)
  • Dual-mode SoundTraxx Digital Command Control sound decoders (DCC version only)
  • Etched, see-through walkways
  • Front and rear speakers (DCC sound version only)
  • McHenry scale magnetic knuckle couplers at correct height
  • Minimum radius: 28"
  • Two flywheel-equipped five-pole skew-wound motors
  • Weight: 1 pound 9 ounces
  • Wire grab irons

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