Union Pacific engine #906, ca. 1900

At the beginning of World War II, the UP developed the 4000-class Big Boy steam locomotives, the biggest and most powerful engines that represented the climax of steam technology. At top speed they could reach 80 miles per hour. The power of the Big Boy engines surpassed the already advanced 9000-class steam locomotive, which had the capacity to pull trains at a swift 50 miles per hour. The 9000-class boasted a very long wheelbase and served the route from Iowa to Green River, Wyo. In 1948, UP began using gas turbine electric locomotives on some trains.