LOCOMOTIVE FEATURES:
• NP and SP&S road specific details
- Cast pilot with fold-down coupler
- Enclosed cab with side entry doors, fixed roof hatches and simulated diaphragm
- Smokebox-mounted air pumps
- Two sizes of wheels in the trailing truck
- NP-style expansion link, which supports much of the valve gear
- NP specific welded coal
- SP&S specific welded oil
• Fully-assembled and ready-to-run
• Boiler backhead with printed manual controls
• Individually applied piping, valves, generators, etc.
• Correctly operating eccentric cranks
• Illuminated LED directional headlight and back-up light in the tender
• Illuminated number boards
• Five pole, skewed armature can motor with flywheels
• Pivoting front and rear engines for negotiating 11” radius curves
• Current pick-up on all driver and wheels
• 8-pin connector plug between loco and
• McHenry operating knuckle couplers installled
• Minimum radius: 11”
• Recommended radius: 15”

PROTOTYPE HISTORY
The Northern Pacific ordered 21 “Challengers” from the American
Locomotive Company in 1936 and took delivery of them during 1936
and 1937. These 4-6-6-4s were designated Z-6 and were assigned road
numbers 5100 through 5120. They had four 23 x 32 cylinders, 69” drivers,
a 250 psi boiler pressure, exerted 104,500 lbs of tractive effort and weighed
624,500 pounds. In 1941, another six “Challengers” came from ALCO.
These were designated Z-7 and they carried road numbers 5121 through
5126. They had four 23 x 32 cylinders, 70” drivers, a 260 psi boiler pressure,
exerted 107,000 lbs of tractive effort and weighed 644,000 pounds. The last
steam locomotives bought by the Northern Pacific were the 20 “Challengers”
it purchased from ALCO during 1943 and 1944. This group designated
Z-8 was assigned road numbers 5130 through 5149. The Z-8 locomotives
were virtually identical to those of Class Z-7. Two oil-burning Z-8s were also
delivered to the SP&S.

CLASS Z-8
This was essentially identical to the 1941 Z-7s. These locomotives were
produced under a wartime mandate to replace certain high-strength steel
components with similar items produced with more common and heavier
steel. Even so, the NP’s diagrams show no difference in the adhesion weight
and just a 1,000 lb difference on engine weight.
The firebox heating surface included 136 sq ft in eight circulators. These also
featured a bearing design that permitted smooth riding at higher speeds.