In order to achieve greater pulling power, the D&RGW further developed their Mallets from the L-95 and L-107 classes to the L-131 class with single steam expansion. With the wheel arrangement 2-8-8-2 and an axle load of over 70,000 pounds it was possible to build a boiler with one of the largest fireboxes at that time. The class designation indicates a starting tractive effort of 131,000 pounds, although the commonly used formula gives about 140,000 pounds. However, the D&RGW determined the practically achievable pulling force to be 131,800 pounds and then rounded this value down.
In 1927, the first ten locomotives were built by ALCO-Brooks. Their service area was at the continental divide, where there was a steepest gradient of 1.42 percent on the east side and of 3 percent on the west side. The train tonnages for one of these locomotives were 3,300 short tons on the less steep side and 1,400 short tons on the steeper side. The drivers apparently complained about excessive smoke production, which became a major problem, especially in tunnels.
When ten locomotives of another series were built in 1930, the dimensions of the power plant did not change. To distinguish them, this time the tractive effort was rounded up and it was called L-132. These were now built at Schenectady and received a new boiler design. Much of the tubes were replaced with the larger flues, which contained additional superheater elements. Like the L-131, they remained in service until the mid-1950s, when the conversion to diesel took place.
