From Helena (MP 0.0), where 2MT from the west end in Yard Limits, with speed restriction 10 mph, the station platform and depot are on the south side of the line, and the former GN r-o-w once ran parallel on the south side of the line, the single track line heads just south of due east, with speed limit rising to 20 mph, past a huge ASARCO smelter on the south side, a yard on the north side of the tracks at East Helena (MP 234.0), just west of where a short branch from Montana City turns away south—all that remains of a once extensive set of branches serving the mines in the mountains to the south—and the main line turns east, single track, CTC, out of Yard Limits, with speed limit 60 mph, and then east-southeast, past Penwell (MP 231.2) and the 7,913 ft. siding at Louisville (MP 227.4).
The line turns curvily south-southeast, descending the west side of the valley towards a river/lake on the east side of the line, past Placer, a detector at MP 221.1, the 6,981 ft. siding at Winston (MP 218.3), Clow, where it comes close to Canyon Ferry Lake, a reservoir on the Missouri River (which flows north along here), Lewark, and turns just east of due south, following the river, past a spur at Continental Lime (MP 206.5), a bridge across the Missouri, which then winds its way along on the west side of the line, the 6,829 ft. siding at Townsend (MP 205.3), a detector at MP 199.8, Holker, and the 8,834 ft. siding at Toston (MP 194.2), turning southeast to come right alongside the east bank of the river, past Brewer, and then curving clockwise to the northwest, counter-clockwise to the southwest and then east-southeast, still following the river, to the 8,574 ft. siding at Lombard (MP 174.9, where the speed limit drops to 25 mph, and the Milwaukee Road's electrified Pacific Extension through this area once came in from the east, crossed the river on a still-extant girder bridge, and headed south on the far side of the river, in the opposite direction along the river from the ex-NP line used by MRL (i.e., heading west)!
The ex-NP line continues southward along the east bank of the river, turning south-southwest past a spur at Stanley (MP 183.1), and then south, past the 8,835 ft. siding at Clarkston (MP 178.8), where the speed limit is 60 mph, and Rekap, turning west-southwest, past a detector at MP 164.6, where the speed limit is 45 mph, northwest, and then counter-clockwise to south, still following the east bank of the river, south-southwest past the 4,943 ft. siding at Trident (MP 170.5), and then turning southeast near Three Forks where the Gallatin River joins with the Madison and Jefferson Rivers to form the Missouri River, which flows northward from this location, following the north bank of the Gallatin River past Carpenter, and then south to the 7,764 ft. siding at Logan (MP 164.6), where the speed limit is 25 mph, and a branch that is the residual trackage of the former NP line over the now-closed Homestake Pass to Butte, trails in from the west, as the main line turns curvily east.