UP Steam Power at Speed Pack

-----===== HISTORY =====----- In the 1920s, the Union Pacific Railroad ordered the TTT-6 Class, a 2-10-2 steam locomotive with the uncommon Young valve gear, which was used to pull heavy freight trains up the demanding 1,14% grade of the Wasatch Mountains. The locomotives were quickly overcome by increasing train length and weight, and doubleheading became frequent. The UP, not keen on the added costs of maintenance and operation, decided they needed more. One solution was an articulated locomotive, with 2 sets of driving wheels, one of which was hinged. While these locomotives were strong, they hammered the tracks heavily at high speeds, so they had to be run slow. The railroad instead went with a rigid frame locomotive, the 9000 Class, delivered in 1928. Although successful on flat country for the most part, its 4-12-2 wheel arrangement could not handle the tight curves of the Wasatch grade or Sherman hill. In the mid 1930s, the UP was still looking for powerful locomotives capable of tackling the harsh grades of their Overland route, so they, along with ALCO, returned to the articulated type. While the 'Mallets' of yesteryear reused the steam from the rear set of cylinders to the front set at a lower pressure, this new locomotive would use the steam once in each of its 4 sets. This breakthrough, along with their 69 inch (1. 75 meters) diameter wheels, meant they could run at high speeds without as much damage to the track. The resulting locomotive was denominated 'Challenger'. The UP, and several other railroads, would order several Challenger types. Suitable for both freight and passenger service, they were considered by some to be the ultimate steam locomotive. But even this was not enough power to conquer the Wasatch grade, so the railroad would order something even bigger. They chose to enlarge the Challenger type, from a 4-6-6-4 wheel arrangement to a 4-8-8-4. This locomotive, nicknamed Big Boy, was one of the largest successful steam locomotives ever made. T