To add a bit about the ex GN line from Helena to Butte alongside I-15 that the orginal poster mentioned.

The GN built this line to tap the mines in Butte before completing the line to the coast. The orginal GN went from Minneapolis to Butte via Havre and Great Falls. While the intention was to build west eventually, the line to Butte would provide needed revenue in the meantime. The GN was not a land grant railroad and thus was built with it's own (investor's) funds. Just west of Havre is Pacfic Junction where the Butte line diverges from the mainline to Seattle. The name gives a clue as to which line was built first.

Shortly after the BN merger a connection in the form of a wye was built west of Helena where the GN Great Falls line crossed the NP mainline. This was to allow trains on the former GN to use the NP Mullan Pass route to Garrison, then diverging to Butte on the former NP Homestake mainline (used by the North Coast Limited and later Amtrak) and a small amount of freight. This allowed the abandonement of the ex GN line south of Basin where talc loading continued although that was always intended to be a short term operation. BN built a connection to the ex GN from the ex NP just west of East Helena and most of the GN trackage through Helena was abandoned.

Before the talc loading ceased at Basin the major floods of 1981 washed out many sections of the line (as well as many sections of the ex NP mainline between Eliston and Drummond) forcing the early closure of the line. With the line abandoned I-15 could be completed between Boulder and Butte without regard to avoiding the RR line.

The short answer to the question is that the line was ex GN (Montana Central as built but owned by GN) with the portion south of Basin abandoned in the early '70's by BN and the Montana City to Basin portion abandoned by BN in '81. East Helena to Montana City was operated by MRL but I don't know if the cement loading is still taking place.

Highlights of the line were the Amazon tunnel, Wickes trestle and the 3 percent grades into Butte. It is the only rail crossing of the Continental divide to be abandoned in MT. The GN used to assemble colorful (orange and/or blue) sets of 6 high-nose GP20's to assault the grade in the '60's.