Numbers 200 to 209 were built by Schenectady in 1909 and 1910 as 0-6-6-0 all-adhesion Mallets. Their special task was to haul rotary snow plows at speeds as low as 4 mph without stalling. Railway Gazette once wrote that this task had previously been taken over by up to five Consolidations and now one of these Mallets was supposed to accomplish this task alone. For the rest of the year, they were mainly used in regular pusher service. As early as 1912, they received a leading axle and a mechanical stoker to improve their usability in freight service. Now they were perfect to haul heavy trains through the mountains with 20 mph, with a top speed of 35 mph. 

In 1913, the railroad became the Denver & Salt Lake. In the same year they ordered numbers 210 to 216 which had a leading axle and a stoker from the start, and additionally were superheated. 210 was lost in a wreck in 1924, but the rest of the class was rebuilt in 1926. The first batch got superheaters of 1,249 square feet, but the second one retained their superheaters of 1,046 square feet. Additionally, thermic syphons and Sellers exhaust injectors were installed. In 1931 the railroad was taken over by the D&RGW and these locomotive became class L-77. They were retired between 1947 and 1952.