One of two classes of 4-6-6-4 “Challenger” locomotives ordered by the DRGW was the L-105 introduced in 1938. It was designed for mixed service on grades in the Rocky Mountains with up to three percent and sharp curves. For these long grades, it had a huge sandbox mounted in front of the smoke box. The 105 in the class designation denoted the starting tractive effort in thousands of pounds. 

The first ten were delivered by Baldwin in 1938 and got the numbers 3700 to 3709. In 1942, five more were ordered which got the numbers 3710 to 3714. They had the same basic dimensions, but got fewer tubes and flues. A typical line the L-105 was running on went over Soldier Summit in Utah. There they hauled 2,750 tons and were assisted by helpers on the steepest portions. The town called Helper was one of those places where the helpers were coupled to the train. They were retired and scrapped between 1951 and 1957.