From the record
“The Northern Pacific Railroad Completion Site is the location of the golden spike ceremony for the completion of the Northern Pacific Railway (NP) in 1883. The site is located near Gold Creek in Powell County, Montana, off Interstate 90, approximately 59 miles (95 km) southeast of Missoula and 40 miles (64 km) west of Helena.”
Wikipedia article Northern Pacific Railroad Completion Site, 1883, CC BY-SA 4.0.
One of 20 National Register listings published here for Powell County · listed in 1983 · category: Site.
Sources: the Wikipedia article · the case file at the National Archives · the National Register database, National Park Service · the Wikipedia article