Butte, Montana was known as " The Richest Hill on Earth ", and for good reason. Many millions of dollars worth of Copper, Silver, and other minerals were mined there, enough to support 4 major railroads. Supposedly Butte Hill has a hundred miles of tunnels under it. The GN line from Helena was quite a nasty line to operate, with several steep grades and much curvature, yet the line hosted a daily freight right up to the BN Merger. The line hosted solid lashups of GN GP - 20's.
BTW, though The Milwaukee Road built the beautiful depot in Downtown Butte, it was a stub-end set-up, requiring a back-up move to or from the mainline. As noted above, Milwaukee built a new, smaller depot on the mainline Southwest of town in the mid - 1950's.