Union Pacific 'Big Boy' Project Information

(Products bearing Union Pacific marks are made under license from Union Pacific Railroad Company)

Kohs & Company proudly presents our most anticipated O scale locomotive project ever, the Union Pacific 4884 class 'Big Boy'. This locomotive is the prototype by which all other large road engines are judged and our plan is to make our Kohs & Company representation of this prototype the model by which all others will be judged, starting with the specifications presented below.

The specifications as listed represent our starting point, not where we will conclude our efforts. Each specification mentioned has already been realized with a previous model and all of them are embodied in our Union Pacific Challenger models, so it is not a matter of wishful thinking for us. As we have routinely done with each project, we expect to surpass all of our previous projects with features, detail and overall quality, we plan to deliver more than promised. This is only made possible by continually upgrading our production technology just as we have our model technology. Although our models will remain always hand-built, the production of component parts relies heavily on the very latest in 3D solid modeling computer design (which has now replaced the usual early sample model), CNC machining and state-of-the-art Rapid Technology for pattern making.

There were a total of twenty-five 4884 class locomotives built in the two production orders contracted for. While there were important differences between the two orders in the details of construction, there were only two significant visual differences; the cooling coils present on the front pilot deck of the first production order and the two different tender versions used for each order. There were additional innumerable subtle visual differences including the routing of piping, secondary air vents on the fireboxes and the main air reservoirs, our plan is to capture all of these differences with the versions selected for production. The matter of tender versions is explained on the Tender Information separate page.

As the 'Big Boys' were put into service there were modifications made based on the initial experiences of the road crews, these modifications will be included in our early-service models, divided into versions #1 and #2 based on their prototypical production order. The most visible modifications were those made to the secondary air vents on the firebox. The later development of this prototype class closely paralleled that of the 'late' Challengers, our last project. Since we are limited here in the details that can be presented, I would suggest visiting the project web site for our Challenger Project to gain a better understanding of the detail evolution of our Big boy. The listing at the right highlights the areas of most significant change during the later service years and again, these areas of detail will be correctly modeled based on our in-depth research.

The photo of Big Boy 4019 shows the only member of the class that received smoke-lifters, commonly referred to as 'elephant ears'. The smoke-lifters were originally developed when the late Challengers were converted to oil-fired operation. 4019 had the smoke-lifters installed during early service and as best we can determine kept them well into the final service years. Had this been only a short-term experiment, it would have fallen into our 'anomaly' category of unimportant situations, but the length of service in this configuration and the interest in similarly configured Challengers made this a must include variation. This variation is our version #5 model and the quantity will be limited.

As previously mentioned, tenders are certainly part of the configuration variables even though with most previously produced models it has not been a serious factor. The transition of tenders through their service life certainly provides some of the most interesting detail. Tender Information.

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  • Custom Built DCC Control, Sound and Lighting System
  • True Digitized Prototype Sounds not synthesized
  • Custom designed Speaker System with Multiple Speakers
  • Twin Custom Wound Ball-Bearing Equipped Pittman Motor
  • Purpose Designed Ball-Bearing Equipped Drive Mechanism
  • Scale Prototypical Stainless Steel Tired Wheel Sets with 68" operating radius
  • Nickel Silver Rods and Valve Gear
  • Full Under-body Detail on Locomotive
  • Full Under-body Detail on Tender with no compromise for the sound system
  • Prototypically built and mounted Running Boards
  • Punched Rivet Detail for maximum fidelity in Rivet Shape and Sheet Metal Contours, particularly on the Boiler Jacket and Tender Tank
  • Fully Equalized and Sprung Suspension with Fabricated Leaf Springs on both the Locomotive and Tender
  • 68 Ball-Bearing Units used throughout the model
  • Ball-Bearing Equipped Side & Main Rod
  • User Controlled Electronically Operated Valve Gear on both Engines
  • Full Cab Detail: Wood Liner, Brass Gauge Bezels & Lenses
  • Real Glass Window Glazing
  • Prototypical Operating Buffer Assembly between Loco & Tender
  • All Hatches and Compartments open to reveal full inside detailing
  • Real Wood Decking on appropriate Tender Versions
  • Prototypical Hollow Core Boxpok Drivers
  • Prototypical Hollow Drive Axles


Version #1 'Early Service' - First production order, road numbers 4000-4019 paired with 25-C-1 tenders.

Version #2 'Early Service' - Second produciton order, road numbers 4020-4024 paired with 25-C-4 tenders

Version #3 'Late Service' - First production order, road number 4000-4019 paired with modified tenders, type to be determined by the railroad assignment in late service.

Version #4 'Late Service' - Second production order, road numbers 4020-4024, paired with modified 25-C-1 tenders.

Version #5 'Late Service' - Coal fired road number 4019 with smoke lifters, the only Big Boy so equipped, paired with a 25-C-1 tender. This model version will be limited in number.

To view the differences between the two tender types built for the two production orders please refer to the Tender Version page by clicking HERE.