D&RGW 50, a tiny 2-axle, 30-ton Davenport diesel, was the only narrow gauge diesel locomotive that the Rio Grande actually owned. (They tried several Army diesel on the narrow gauge lines, but never owned any of them.) The unit, purchased from the Sumpter Valley in 1963, produced 160hp from its Caterpillar D17000 engine, with power coupled to the wheels through a mechanical transmission.

The unit was used to switch the Durango yard from its purchase in 1963 until the abandonment of the San Juan Extension in 1970. At that point, it was sold to the Roaring Camp & Big Trees near Santa Cruz, California. Last running in the early 1970s, the engine was eventually sold to the Colorado Railroad Museum in 1984.

The engine is preserved at the Colorado Railroad Museum today, and was returned to operation in June 2009.