Big Boy is the popular name of the American Locomotive Company 4000-class 4-8-8-4 articulated, coal-fired, steam locomotives manufactured between 1941 and 1944 and operated by the Union Pacific Railroaduntil 1959.


The Big Boy fleet of twenty five locomotives were used primarily in the Wyoming Division to haul freight over the Wasatch mountains between Green River, Wyoming and Ogden, Utah. They were the only locomotives to use a 4-8-8-4 wheel arrangement consisting of a four-wheel leading truck for stability entering curves, two sets of eight driving wheels, and a four-wheel trailing truck to support the large firebox.


According to Union Pacific senior manager of Heritage Operations Ed Dickens Jr., the 4-8-8-4 series originally was to have been called "Wasatch". One day while one of the engines was being built an unknown worker scrawled "Big Boy" in chalk on its front. With that, the legendary name was born, and has stuck ever since.


The Big Boy 4014 is currently in restoration and will be in operational condition in 2016. As part of the restoration process, Union Pacific will convert 4014 from coal to No. 5 oil firing.