From the record
“The Durango & Silverton Narrow Gauge Railroad Company, often abbreviated as the D&SNG, is a 3 ft (914 mm) narrow-gauge heritage railroad that operates on 45.2 miles (72.7 km) of track between Durango and Silverton, in the U.S. state of Colorado. The railway is a federally-designated National Historic Landmark District and was also designated by the American Society of Civil Engineers as a National Historic Civil Engineering Landmark in 1968.”
Wikipedia article Durango and Silverton Narrow Gauge Railroad, CC BY-SA 4.0.
One of 45 National Register listings published here for La Plata County · listed in 1966 · category: District · a National Historic Landmark.
Sources: the Wikipedia article · the case file at the National Archives · the National Register database, National Park Service