The line you saw over Homestake Pass is a part of Montana Rail Link, however it has been out of service for many years. It was built as part of Northern Pacific's original line to the Pacific Northwest. As you have noted, the line is quite twisty and slow, with a curved steel trestle on it as well. NP would later build a better line between Logan and Garrison through the State Capitol of Helena; this line is currently a part of the MRL mainline. After the Helena line was built, the Butte line remained in use primarily for passenger trains. I rode over this line on Amtrak's North Coast Hiawatha in August, 1979, just 2 months before this train was discontinued.