By 1941, Union Pacific declared a new Type of Locomotive. They called these Articulated Giants. Three "Articulated Giants" Type models were made. The Union Pacific officially called all three the "Behemoth Fleet".
The Behemoth Fleet consisted of: Union Pacific ALCO 4000 4-8-8-8-4 "Big Boy" Union Pacific ALCO 4-6-6-4 Challenger "Dark Horse" Union Pacific ALCO 4-8-4 Northern "UP North"
The Union Pacific ALCO 4000 4-8-8-8-4 "Big Boy", most commonly mistaken for 4-8-8-4, was the largest of the Behemoth Fleet. Also the largest Steam Locomotive on Earth. 1941, 20 were produced, by 1944 a total of 25 were made. The Big Boy was such a powerful train, it had more horsepower than 70 Formula 1 Race cars combined. One fact about the Big Boy, because of it's power, it actually distributed a 4.1 Earthquake as it ran at HighBall. HighBall is when a Steam Locomotive sets maximum Pressure to give all power, reaching the maximum speed almost instantly.
The Union Pacific ALCO 4-6-6-4 Challenger "Dark Horse" is the younger brother of the Big Boy. The Challenger isn't as powerful, because it was designed for a minimum amount of cargo. As well as short distance travel, the Challenger eventually started having problems. This ended it's production.
The Union Pacific ALCO 4-8-4 Northern "UP North" is the Big Boys little sister. Much less power, and much less size. This train was mainly used during WWII for supply transfers. It didn't see as much action as the other two, but it was unique for it's design.
Over the years the Union Pacific Big Boy was the big Icon of Union Pacific, it was their FlagTrain. Eventually demands for this train ended. There were 25, now only 8 remain preserved, the rest were scrapped. Not much exists of the Challenger and the Northern.
The Behemoth Fleet, and the Big Boy once called "Legend on Rails" now exist only in history books for Conductors, almost completely forgotten.
All official BluePrints for the Behemoth Fleet are perfectly preserved, the Big Boy, Challenger, and the Northern are still restorable with the preserved BluePrints.