Union Pacific No. 4000 is a four-cylinder articulated 4-8-8-4 Big Boy-type steam locomotive owned by the Union Pacific Railroad. This locomotive was built by Alco in 1941 along with the other Big Boy locomotives to address the problems concerning the preceding 4-6-6-4 Challenger class locomotives.
It ran in service until July of 1959, when it was retired along with the other Big Boy Locomotives. It was sold for scrap.
- No. 4000 is the prototype of the class.
- The 4-8-8-4 class series, originally rumored to be called the "Wasatch", acquired its nickname after an unknown worker scrawled "Big Boy" in chalk on the front of No. 4000, then under construction as the first of its class.
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