In the 1930s, with freight traffic increasing, the Union Pacific Railroad was struggling to get trains over the rugged grades of the Wahsatch Mountains. To stay competitive, a more powerful locomotive was needed to speed up the railroad and to reduce the rising cost of helpers and extra trains. An original design of 4-12-2 was proposed, but after much deliberation, an arrangement of 4-6-6-4 was finalised and the Challenger was born. In August 1936, number 3900 was the first to enter revenue earning service with a refrigerator train from Council Bluffs. A total of 252 were eventually operating around North America.