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HO - Steam Loco Kits
Union Pacific Challenger 4-6-6-4

All loco kits, tender kits and superdetail kits are sold out and discontinued.
Most parts are available for repairing old locos.


The 4-6-6-4 class, original Challenger was designed by Otto Jabelmann of the Union Pacific and first built by Alco for UP. Approximately 230 Challengers were built nearly alike, differing only in their steam pressure, cylinders, and boilers. All Challengers had either 69" or 70" drivers and were rated at 94,400 pounds tractive effort on the Delaware Hudson to 106,900 pounds tractive effort on the Northern Pacific.

The 4-6-6-4 was often used for passenger service, but its main function was carrying heavy, fast freight. It could average speeds of up to 70 miles per hour.

The original Challengers had 21" x 32" cylinders, 69" drivers, 255 pounds steam pressure and weighed 566,000 pounds.

The original Union Pacific Challengers were numbered from 3900 to 3939 when they came from Alco, but were renumbered to 3800 to 3839 in 1944 in order to allow space for use on later engines.

Alco modified the Challenger starting in 1942 and ending in 1944, making a total of 105 new Challenger locomotives. These improved locomotives had attached front engines. Springs and equalizers took up all irregularities in the track to keep the train in equilibrium. This better balance allowed the new Challengers to reach speeds of up to 70 miles per hour or more.

Boiler pressure was increased to 280 pounds, allowing for smaller cylinders. Drivers were still 69", but the total wheelbase was made 5 1/4" longer. The engine now weighed 627,000 pounds and the tractive effort increased to 97,350 pounds.

The engines were numbered in three groups, 3950 to 3969, 3975 to 3999, and 3930 to 3949, with the overlap mentioned earlier above.

Twin stacks were used, large sandboxes were added, as well as 14-wheel bed type tenders of the centipede pattern. Elesco exhaust steam injectors were mounted on the left side of some engines, and were hidden on the inside of others. The pilot was now made of cast steel, and the boilers were made of Bethloc deoxidized steel boilers, replacing the original boilers, which were made of manganese.

Some engines that had been originally intended for the Union Pacific were diverted to the Denver and Rio Grande Western. These were numbers 3800 to 3805. They did not correspond with other Challengers made by Baldwin and were sold to Clinchfield. Clinchfield then had two types of Alco Challengers corresponding to the two types on the Union Pacific.

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#100300 Locomotive Kit    
#100325 Super Detail Kit (over 120 brass parts)  
#500305 Assembled valve gears wit eccentrics  
Painted Engineer & Fireman


Builder ALCO
Class Railroad No.'s 3900 (Union Pacific)
Tractive Force 97,350 pounds
Drivers 69"
Weight on Drivers 406,200 pounds
Weight on Front Truck 101,700 pounds
Weight on Trailing Truck 126,700 pounds
Total Engine Weight 634,500 pounds
Total Engine Wheel Base 60' 4-1/2''
Steam Pressure 280pounds
Height from rail to top of Smokestack 16' -3"
Drivers 69" Box Pok
Gear Ratio 31-1
Power Bowser DC-71 motor
Overall Length:
(Engine & SP-1 tender) 16"
w/ long haul 17 1/2"
or 14 wheel tender

Total Engine Weight 1 lb. 15 1/4 oz.
Total overall length of engine
(Pilot to drawbar screw) 10.413"
Height 2.238"


(From inside of Outside Rail)
18"Radius Curve 1.13"
20"Radius Curve .99"
22"Radius Curve .87"
24"Radius Curve .83"
26"Radius Curve .77"
28"Radius Curve .71"
30"Radius Curve .67"

NOTE: If curves are given the clearance as outlined in NMRA standards, no difficulty with overhang will be encountered.


Tenders for the Challenger

#150001 SP-1 Semi-Vanderbilt Tender Kit
#150650  Long Haul Tender Kit
#150681 Electrical Pickup Kit for Con-Cor tenders 

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Kit Photos

Assembly Instructions 

Photo of Superdetailed Model

Prototype Photos