During World War II in the 1940's, railroads in the US were in dire need of larger steam locomotives to deliver long and heavy trains (which became longer and heavier due to the stock market boom as a result of the US recovering from the Great Depression; aside from having more materials needed to built vehicles and weapons to supply troops in combat), to whom the economy skyrocketed after the Great Depression, and many companies relied on trains to deliver their goods.
Hence, railroads like the Union Pacific needing larger locomotives to haul such long and heavy freight trains through their long routes. Which meant the request from ALCO to design a much larger version of their 'Challenger' type of 4-6-6-4 articulated steam locomotive for conventional horsepower.
The 4000 Class dominated the steam locomotive market by having less wheels and more power and length compared to other Mallet types of steam locomotives (such as the "Erie Triplex" Class from the Erie Railroad), which meant that they were easier to maintain, yet they were very expensive to operate because of how much water and coal they consumed, as well as how much time, effort, and money used to mold the metal and other materials for the steaming giants (yet, more purchases led to ALCO's large earnings).
Once they were purchased by the Union Pacific; and when they were still in production, most railfans, train enthusiasts, locomotive builders, and railroad employees nicknamed the giant type of steam locomotive the "Big Boy". (yet the name; however, was rumored to have began as a simple nickname stenciled in the boiler of an ALCO employee during the construction of the first one built in early-1941). But their times hauling trains on the Union Pacific system unfortunately came to an end during the 1950s, when the GE GTEL Gas Electric Turbine super-powered diesel turbine locomotives began to replace the Big Boy's during the end of the Steam Era, however in their last few years, they were put onto reserve service until their official decommission in 1962. Seventeen engines succumbed to the cutter's torch; surprisingly - for such a small number built - eight of the 25 beasts built survive, all which are currently on static display in various museums throughout America, except for the 4014, which is now at UP's Cheyenne, WY steam facility, under restoration to operating condition.
- Apparently, the original name of the Big Boy was supposed to be "Wasatch".
- The Big Boy has more steam valve controls than any other type of steam locomotive.
- Most average people and train enthusiasts often confuse the Challenger and Big Boy for one another. The main differences are the position of the boiler number plate, and the wheel arrangements.
- No Big Boys are currently operational. However, number 4014 is currently being restored in the Union Pacific's Steam Shops; she is expected to return to steam in 2019.
- Big Boys were rarely occasionally assigned to passenger service.
- Most crew members who drove Big Boys often used auger drills to drill the coal stored in the tender because of how long it often sat due to the size of the tender. Hence, coal being refilled every month or so.
- The Japanese anime TV series "Galaxy Railways" features a space train named Big One, a very stylized depiction of a Big Boy.
- The Big Boys smokestack were actually two smokestacks combined and covered with smoke deflectors.
- There has only been one wreck in Big Boy history, involving a lazy switch track controller and 4005 pulling a heavy freight train of boxcars. Thankfully, the 4005 was repaired and returned to service.
- Big Boys 4004, 4005, 4006, 4012, 4014, 4017, 4018, and 4023 have survived into preservation.