The Railfan Nation named this 6 out of the top 50 pictures in the group for the year 2012. Thank you to everyone!

The biggest steam locomotive in the world is the Union Pacific Big Boy, designed by Otto Jabelmann and built by ALCO in 1941 to pull long, heavy trains over the mountains. The class was originally to have been named "Wasatch" for the mountain range. However, during construction, a worker scrawled "Big Boy" on the body of one of the big engines, and the UP decided to call the engines Big Boys.

The UP was obsessed with big engines, which would let them run longer and fewer trains on the steep slopes, and the 4-8-8-4 Big Boy was the acme, the zenith, the biggest Big Engine of all. Each of the 25 Big Boys (Nos. 4000-4024) was nearly 140 feet long, as tall as a house, and weighed over 600 tons. In other words, the locomotive had the size and weight of a small ship. The firebox burned coal fed by a mechanical stoker, and a centipede tender carried the coal and water supply for the long boiler. The 4-8-8-4 articulated Mallet configuration, in which the front of the engine could pivot, let the Big Boy go around curves in the mountains. The Big Boy was not only powerful enough to pull 3000 or 4000 ton trains, but was also very fast for its size, able to reach 80 miles per hour. This became very useful in World War II, when the Big Boys had to haul long military trains across the country.

The Big Boys were retired in the early 1960s, and No. 4014 is preserved at the Railway Locomotive Historical Society ("Rail Giants") at the Fairplex in Pomona, California. Given the sheer size of the Big Boy, which could only be turned on two long turntables, there are no plans to restore it to operating status.

Happy 150 years, Union Pacific!

Over 4000 views in the first day? And hundreds of favorites? Thanks everyone! I've never had a picture be this successful before!

PS: I know there is a lot of debate over this, but I think the Big Boy is generally recognized as the overall biggest steam engine when you average out the size, weight, and power.

PPS: Thank you for everyone who came and viewed this picture, as I have just gotten over 10,000 views on it!

PPPS: According to Trains Magazine, the Union Pacific Railroad wants to take No. 4014 and restore it to operating condition in time for the 150th anniversary of the transcontinental railroad in 2019. If that is true, then the Big Boy will ride again!