The class M-67 were the first Mountains introduced into service by the D&RGW. In 1922, the press presented them as “largest passenger locomotives in the world”. To enable high average speeds over mountainous lines, they had drivers of only 63 inches or 1,600 mm. Unlike most Mountains in North America they were not connected at the second, but at the third driving axle like most Mikados. The driving axles had a wheel base of 17 ft 3 in, but a flexible bearing in the lead driving axle led to a rigid wheel base of only 11 ft 2 in.

The first ten were built by ALCO-Brooks in 1922 and numbered 1501 to 1510. They received their designation M-67 due to their tractive effort of around 67,000 pounds. In the following year, No. 1511 to 1520 were built with a trailing truck booster and designated class M-78 due to their higher tractive effort. The last ten built in the same year and numbered 1521 to 1530 again had no booster and were put into class M-67. In total they were 1,600 pounds heavier than the first ten.

Over the years these locomotives received some modifications to increase their power and efficiency. These included thermic syphons and Elesco feed water heaters. The twelve-wheel tenders already had a large coal capacity of 20 tons at the beginning, but that was later increased to 25 tons. The first M-67 was retired in 1949, but the first M-78 only followed in 1952. The last member of both classes was gone in 1955.