The Union Pacific Big Boy is a type of steam locomotive built by the American Locomotive Company between 1941 and 1944. It was designed for hauling heavy freight across the steep grades of the Wasatch Range in the western United States. The Big Boy is notable for its massive size, featuring a 4-8-8-4 wheel arrangement, which means it has four leading wheels, eight driving wheels, and another set of eight driving wheels. With a total of 25 units produced, the Big Boy locomotives were among the largest and most powerful steam engines ever built. They were retired from service in the 1960s as diesel locomotives became more common. Today, a few Big Boy locomotives are preserved and displayed in museums, while others are operational, showcasing the engineering marvel of their time.