 The line from Hakodate was duplicated for 8 km to Kikyo 1941-44, with the 9 km Ishikura to Nodaoi section double-tracked in 1945.The 5 km Kikyo to Nanae section was double-tracked in 1962, and the doubling effectively extended 13 km to Ōnuma in 1966 with the construction of a new alignment for northbound trains to avoid the 1 in 50 (2%) grades between Oshima-Ono and Ōnuma.Doubling of the 41 km section between Nodaoi and Oshamambe (excluding two gaps totalling 8 km) was undertaken in sections between 1965 and 1984, with the 13 km Mori - Ishikura section doubled between 1974 and 1979.