Prior to 1924, the Denver & Rio Grande Railroad? and its predecessors used a class number scheme based solely on thousands of pounds of weight. As an example, a 140,000 pound K-28 would be a class 140, and a ~125,000 pound K-27 would be a class 125.

In 1924, as part of the 1921 reorganization into the Denver & Rio Grande Western Railroad?, the classification system was redone. Steam locomotives were now classed by wheel configuration and thousands of pounds of tractive effort, indicated by a character prefix and a number. For example, K-36 would indicate a 2-8-2 with roughly 36,000 pounds of tractive effort.