The Denver & Rio Grande Western Railroad bought ten 4-6-6-4s from the Baldwin Locomotive Works in 1937 to speed up its freight trains. These locomotives were designated Class L-105 and were given road numbers 3700 through 3709. They had four 23 x 32 cylinders, 70" drivers, a boiler pressure of 280 psi, a tractive effort of 105,000 lbs and weighed 641,000 pounds.

Five additional Class L-105 "Challengers" arrived from Baldwin in 1942. These were similar to the Class L-105s bought in 1938 and were assigned road numbers 3710 through 3714.

In need of more locomotives to handle war-time traffic, the D&RGW ordered six more "Challengers". The War Production Board allowed six ALCO-built 4-6-6-4s to be delivered in 1943. The D&RGW did not like them, and leased them instead of buying them. These locomotives were designated Class L-97 and had four 21 x 32 cylinders, 69" drivers, a boiler pressure of 280 psi, a tractive effort of 97,350 lbs and weighed 627,000 pounds. In 1947, diesels were replacing many of the D&RGW's steam locomotives and the six Class L-97 "Challengers" were transferred to the Clinchfield Railroad.

There are no surviving D&RGW "Challengers".