On 19 November 2016, JR Hokkaido's President announced plans to rationalise its network by up to 1,237 km, or ~50% of the current network, including the proposed conversion of the Senmo Main Line to Third Sector operation, but if local governments are not agreeable, the line will face closure.
In 1887 a private 41 km 1067mm gauge line was opened from a sulphur mine at Atsanobori to a refinery at Shibecha, 48 km north of Kushiro. Known as the Kushiro Railway, it closed 9 years later when the sulphur was mined out. 35 years after it closed the 17 km section of the Senmo line between Mashu - Shibecha was built on the formation of the Kushiro Railway.
Construction of the Senmo line started from both Asashiri and Kushiro, with the northern portion opening to Satsutsuru in sections between 1924 and 1929, and the southern portion opening in sections to Kawayu between 1927 and 1930. The Satsutsuru - Kawayu section opened in 1931, completing the line.
Freight services on the line ceased in 2002.
- A 39 km 762 mm (2 ft 6 in) gauge line opened to Nakashibetsu in 1932. In 1936 it was replaced by a 1,067 mm (3 ft 6 in) line, which was extended 40 km to Shibetsu, operating until 1989. A branch from Nakashibetsu connected to the Nemuro Main Line at Attoko between 1933 and 1989, the two branches being known collectively as the Shibetsu Line. A total of 9 separate 762 mm (2 ft 6 in) gauge development lines connected to stations along the Shibetsu Line were built between 1930 and 1963, the last closing in 1971.
- Another 762 mm (2 ft 6 in) gauge system connected the Numahoro District to Shibecha, consisting of a 23 km 'main line' opened 1955, and a 6 km branch opened 1966. Both closed in 1971.