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



History:  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.

Complete Kit and Accessories:  The locomotive and tender complete kit has all of the parts to build a working model. You may wish to add your own details or purchase our "Super Detail Kit" if available. Some models have an optional painted engineer and fireman, smoke unit kit and other assemblies available as optional equipment to compliment your kit-building needs.


Kit Components and Replacement Parts:  The following parts are listed in order to identify parts in your kit or to replace parts that have been lost, damaged or simply worn out.  This is not a complete list of all of the individual parts available for this model - please call Bowser with your specific requirements if you cannot find the parts you need.  You may also use our on-line catalog to find parts by description.  Please note that part numbers with a price of "Call Us" may be part of an assembly and may not be available as a separate purchase item.


  Prototype 
  Specifications
  Model
  Specifications
  Engine 
  Overhang
  Builder ALCO
  Class Railroad No.'s 3900 (UP)
  Tractive Force 97,350 Lbs
  Drivers 69"
  Weight on Drivers 406,200 Lbs
  Weight on Fr Truck 101,700 Lbs
  Weight on Trail Trk 126,700 Lbs
  Engine Weight 634,500 Lbs
  Engine Wheel Base 60' 4-1/2''
  Steam Pressure 280 Lbs
  Height- 16' -3"
    Rail to Smokestack Top
  Drivers 69" Box Pok
  Gear Ratio 31-1
  Power- DC-71 motor
  Overall Length: 16"
    Engine&SP-1 Tender
    w/ long haul 17 1/2"
    or 14 wheel tender
   Engine Weight:
    1 lb. 15 1/4 oz.
  Overall Engine Length
    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.


Photographs of Super Detailed Model
Photo 1 Photo 2 Photo 3 Photo 4 Photo 5 Photo 6
Photo 8 Photo 9 Photo 10 Photo 11 Photo 12
Photo 13

Photographs of Prototype
Photo 1 Photo 2 Photo 3 Photo 4 Photo 5 Photo 6
Photo 7 Photo 8 Photo 9 Photo 10 Photo 11 Photo 12
Photo 13 Photo 14


Parts List:  List of parts included in the Challenger Super Detail Kit (see parts list above)

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