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Athearn HO scale class Z-8 Challenger

Read this review from the August 2017 Model Railroader
Athearn HO scale class Z-8 Challenger
The tender features an operating light-emitting diode backup light and non-operating marker lights.
Engineer's side of the Athearn HO scale class Z-8 Challenger

A Northern Pacific RR class Z-8 Challenger is available for the first time in HO scale as a ready-to-run plastic model. Produced by Athearn Trains as part of its Genesis series, the model is accurately detailed to match NP and Spokane, Portland & Seattle Ry. prototypes. For Digital Command Control users the 4-6-6-4 comes to life, thanks to a factory-installed sound system.

Our first-run sample features a SoundTraxx Tsunami DCC decoder. According to Athearn, future production runs will be equipped with ESU LokSound Select decoders. Direct-current (non-sound) versions will have 21-pin decoder sockets.

The prototype. American Locomotive Co. (Alco) delivered the first 21 4-6-6-4s to the Northern Pacific in 1936. These were rostered as class Z-6. As with those built for other railroads, the NP Challengers were simple-expansion rather than compound articulated locomotives. With two sets of high-pressure cylinders, the 4-6-6-4s could reach passenger train speeds rather than the lumbering pace of slower, compound “Mallet” articulateds.

In 1941 Alco delivered six more Challengers to NP. Heavier and more powerful than the Z-6s, these were dubbed class Z-7. To deal with the increased traffic caused by World War II, the NP purchased 20 more Challengers dubbed class Z-8 in 1943 and 1944. These were virtually identical to the class Z-7 Challengers. With an engine and tender weight of one million pounds and nearly 107,000 pounds of tractive effort, these class Z-8 Challengers were the most powerful 4-6-6-4s ever built.

Two additional wartime Challengers, added to the NP’s 1944 order, went to the Spokane, Portland & Seattle Ry., a jointly owned subsidiary of the NP and Great Northern Ry. These locomotives were identical to the NP class Z-8s, except that they burned oil instead of coal. (Two NP Z-8s, nos. 5140 and 5148, were also converted to oil in the 1950s after they were assigned to the SP&S.)

The class Z-8 Challengers sped through freight, troop trains, and passenger varnish over main lines with undulating saw-tooth profiles. The wartime locomotives primarily served in the Idaho, Rocky Mountain, and Yellowstone divisions.

All the NP and SP&S Challengers were retired by 1957. None are preserved.

The model. The major dimensions of the Athearn model match prototype builder’s diagrams. The space between the HO locomotive and tender is about a scale foot wider than on the prototype to help the model negotiate tight curves.

The plastic locomotive features well-defined molded detail, including boiler bands, rivets, and tread texture atop the running boards. Separately applied plastic parts include piping, throttle linkage, whistle, and bell. The handrails along the boiler are made of wire.

The front end of the model captures the look of the prototype, with dual air pumps, a low-mounted headlight, and a solid pilot with a non-operating scale-size drop coupler.

The all-weather cab includes an accurately detailed boiler backhead, crew seats, and glazing in all the windows.

The valve gear and siderods are modeled with blackened metal parts. I appreciated that the eccentric cranks are oriented correctly. Each eccentric tilts forward when its associated main crankpin is at the 6 o’clock position.

The tender is also an accurate model of its NP prototype. Its separate detail parts include piping, handrails, and non-working marker lights with red lenses.

All the lettering on the locomotive and tender is correctly placed. The fonts also match those of the prototype.

Mechanism. On a prototype Challenger, the front engine pivots while the rear engine is fixed. On the model, both engines pivot to allow it to negotiate tight-radius curves. The Athearn Challenger will round an 18" curve, but looks much better on broader curves.

The dual-flywheel can motor sits in the middle of a die-cast metal chassis. Worm shafts connect the motor to the second axle on the first engine and the third axle of the rear engine. The metal siderods transfer power to the other driver axles.

The drivers on the third axle of the rear engine have traction tires. The other drivers and eight wheels on the tender’s pedestal truck pick up track power.

The Challenger didn’t have any power loss issues, even when running through yard ladders or crossovers, when I tested it on our staff layout, the Milwaukee, Racine & Troy. On our hill climb test, the Challenger hauled an impressive 33 HO freight cars up a 3 percent grade.

The SoundTraxx Tsunami decoder and two round speakers are housed in the tender. A wiring harness runs from the locomotive to an 8-pin socket on the front of the tender. The locomotive drawbar easily connects to the tender by simply snapping in place.

Operation. During DCC testing the locomotive featured the smooth performance and realistic sounds that are a hallmark of SoundTraxx Tsunami decoders. During testing the model accelerated smoothly from 4 scale mph to a prototypical top speed of 72 scale mph. Setting the decoder to 128 speed steps lowered the starting speed to 2 scale mph.

A printed quick start DCC guide with all the default configuration variable settings is included with the model. More extensive programming guides are available as free downloads on the Athearn website and at

I easily changed the locomotive’s address, adjusted the volume levels of individual effects, and added acceleration and deceleration momentum. I also set up the train brake, which let me use a function button to stop the train independently of the throttle setting.

The decoder’s Digital Dynamic Exhaust varies the intensity of the exhaust sounds according to the motor load and throttle setting. For example, the chuffs get noticeably louder and deeper when the locomotive starts out with a train.

At most speeds the exhaust sounded the correct four chuffs per wheel revolution with the two engines occasionally going out of sync, which realistically simulates one engine slipping more than the other. There isn’t a mechanical cam, but the audio synchronization can be fine-tuned using CV116. The randomness of the engines going out of sync can also be adjusted.

Unlike earlier Athearn releases that used incandescent bulbs, the Challenger features light-emitting diodes (LEDs) for the number boxes, backup light, and dimmable headlight. Out of the box, the headlight and backup light operate according to the locomotive’s direction of travel, but both can be programmed for manual independent control.

Although the decoder is dual-mode, I can’t recommend the Tsunami-equipped Challenger for DC operation. The model didn’t start moving smoothly until I applied 9.5 volts to the rails. At that point it was already traveling at 27 scale mph. By 12 volts it was at its top speed of 73 scale mph. I tried to lower the starting speed by using a DCC system to lower the analog starting voltage (CV63), but this had no effect. A DC (no sound) version is also available.

Whichever version you choose, the Athearn Genesis class Z-8 Challenger has a detail level comparable to a brass model and a smooth mechanism with powerful pulling capabilities. It’s a must-have locomotive for fans of big NP steam.


Price: $629.98 (DCC sound), $529.98 (DC no sound)

Athearn Trains
1600 Forbes Way, Suite 120
Long Beach, CA 90810

Era: 1943 to 1957

Road names (multiple road numbers available): Northern Pacific; Spokane, Portland & Seattle (with oil tender)

▪▪Electrical pickup on 10 locomotive drivers and forward eight wheels of tender pedestal truck
▪▪Five-pole skew-wound motor with dual brass flywheels
▪▪McHenry operating knuckle coupler on rear of tender (at correct height)
▪▪Metal wheels in gauge
▪▪Minimum radius: 18" (22" or greater recommended)
▪▪SoundTraxx Tsunami decoder (DCC version, 1st run); ESU LokSound decoder (DCC version, 2nd run)
▪▪Traction tires
▪▪Weight: 2 pounds, 2.2 ounces (locomotive and tender); 1 pound, 9.1 ounces (locomotive only)

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Plan and design an around the room track plan.

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