 The last steam locomotive hauled regular service ran between Iwamizawa and Muroran in 1975.Growing coal traffic on the Iwamizawa - Muroran section resulted in the double-tracking of the 101 km Higashi-Muroran - Mikawa section in stages between 1920 and 1958.The 7 km Iwamizawa - Shibun section was double-tracked in 1961, but returned to single track in 1994.