From the record
“The Stevens Pass Historic District is an area within a large rectangle 3.2 by 18.2 miles and extends from the Martin Creek Tunnel on the western slope of the crest to the eastern portal of the present Cascade Tunnel above Nason Creek on the eastern slope. The area is ruggedly mountainous and the terrain is covered with timber and granitic outcroppings.”
Wikipedia article Stevens Pass Historic District, CC BY-SA 4.0.
One of 324 National Register listings published here for King County · listed in 1976 · category: District.
Sources: the Wikipedia article · the case file at the National Archives · the National Register database, National Park Service