The Challengers boasted several technical breakthroughs that contributed to their success. Operating with a high boiler pressure of 280 lbf/in2, a feature typically reserved for passenger locomotives such as the FEF Series, they sported 69-inch drivers—impressive mammoth wheels typically found on passenger locomotives. Most freight engines required smaller wheels to generate the necessary torque. Surpassing speeds of 60 miles per hour, which was unprecedented for most other railroads using articulated steam locomotives, became a regular occurrence on the Union Pacific.

During Union Pacific's fourth order in 1943, ALCO built thirty-one locomotives adhering to the same specifications. However, due to the War Production Board's intervention, six completed locomotives were diverted to the Denver and Rio Grande Western Railroad through a lease facilitated by the War Department's Defense Plant Corporation. These locomotives, numbered 3900-3905, formed the Rio Grande's Class L-97. Later, in 1947, they were sold to the Clinchfield Railroad and renumbered as 670–675, becoming the Clinchfield's Class E-3. Unfortunately, these six Challengers were eventually retired in 1953.

Among the surviving Challengers, Union Pacific No. 3985 stands out. Restored by the Union Pacific in 1981, it operated in excursion service as part of the heritage fleet program. Recently, it was donated to the Railroading Heritage of Midwest America organization in 2022, where it is currently undergoing a second restoration. The only other surviving Challenger, UP No. 3977, is on display in North Platte, Nebraska.

• Paragon4 Sound & Control System featuring ROLLING THUNDER

  Integral Dual-Mode Decoder with Back EMF for Superb Slow Speed Operation in DC and DCC

  Precision Drive Mechanism Engineered for Heavy Towing and Smooth Slow Speed Operation

  Operating Smoke Unit with On/Off Switch

  Six Drivers and All Tender Wheels Pick-up Electricity from Track

  Die Cast Locomotive Body with Die Cast Chassis for Heavier Weight and Maximum Tractive Effort

  Rubber Tires on Two Drivers for Additional Tractive Effort

  Premium Caliber Painting

  Prototypically Accurate Paint Schemes and Road Numbers

  Many Separately Applied Details such as Handrails, Grab Irons, Piping, Tender Ladders, Whistle, and Bell

  Prototypical Headlight and Rear Light Operation

  Separately Controllable Cab Light and Lighted Numberboards

  Prototypically Accurate Sounds for the Union Pacific Big Boy

  Operating Microtrains-compatible Couplers

  Will Operate on Code 55, Code 70, and Code 80 Rail

  Minimum Operating Radius: 9.75 inches

  
  

  Broadway Limited Imports' first production run of Challengers features the following notable characteristics:

  Accurate front end modeling, including an appropriate aftercooler system for each class.
  Accurate modeling of the 25-C-100 and 25-C-400 tenders, tailored to match each class.
  An accurately modeled oil tender specific to No. 3985, representing its excursion service.
  One model featuring Rio Grande lettering and paint.
  One model featuring Clinchfield lettering and paint.
  Two unlettered and black painted models—one with wind wings, and one without.

  In addition to the Paragon4 Sound/DC/DCC sound models, we are also offering these locomotives as part of our Stealth series. These are No-Sound / DCC-Ready models that operate on DC out of the box, or a modeler can easily install their DCC decoder of choice. Out of the box, front and rear lights are directional, and other lights are always illuminated with track power. Additionally, some lighting may be missing depending on selected aftermarket decoder the modeler installs. See below for additional model features.

  Industry-leading Model Features:

  Stealth Series - Model is DCC-Ready out of the box. No sound.
  Out of the box, front and rear lights are directional. Other lights are ON with track power.
  Model comes with 8 ohm speaker installed for easy installation of DCC/Sound decoder.
  Precision Drive Mechanism engineered for continuous heavy load towing and smooth slow speed operation
  Die Cast Locomotive Body with Die Cast Chassis for Heavier Weight and Maximum Tractive Effort
  Rubber Tires on One Driver for Additional Tractive Effort
  Premium Caliber Painting with Prototypically Accurate Paint Schemes and Road Numbers
  Separately Controllable Headlight and Rear Light
  Many Separately Applied Details such as Handrails, Grab Irons, Ladders, Piping, Whistle, and Bell
  Operating Microtrains-compatible Couplers
  Will Operate on Code 55, Code 70, and Code 80 Rail
  Minimum Operating Radius: 9.75 inches