Senmō Main Line

The corridor has a faint prehistory. In 1887 a private 1,067 mm gauge line, the Kushiro Railway, was opened to carry sulphur from a mine at Atsanobori to a refinery at Shibecha, about 48 km north of Kushiro; it closed some nine years later when the deposit was worked out. Decades afterwards the state railway, building south from the Shibecha direction, reused part of that old formation, but the Senmō line proper was a fresh project begun in the 1920s and constructed from both ends at once — southward from Abashiri and northward from Kushiro — to be joined in the middle.

The northern half opened first, and under a different name. On 15 November 1924 the Abashiri Main Line was extended from Abashiri (its first-generation station) to Kitahama, a stretch of about 11.6 km along the drift-ice coast. The rails reached Shari on 10 November 1925 and pushed on to Sattsuru on 14 November 1929, carrying the Abashiri Main Line name as they went.

The southern half came from Kushiro and was called the Senmō Line. It opened from Kushiro to Shibecha — roughly 48.1 km — on 15 September 1927, following in part the route of the long-defunct Kushiro Railway. The line then climbed inland toward the Kussharo caldera, reaching Teshikaga on 15 August 1929 and Kawayu (today Kawayu-Onsen) on 20 August 1930.

The two halves were finally joined on 20 September 1931, when the gap between Sattsuru and Kawayu — about 22.8 km — was filled in. With through running established between Kushiro and Abashiri, the whole route was operated as one line. On 29 October 1936 it was renamed the Senmō Main Line, the name it has carried ever since, with "Senmō" formed from characters for Kushiro (釧) and Abashiri (網).

The line passed through the twentieth century as a Japanese National Railways (JNR) rural trunk route. When JNR was broken up and privatised on 1 April 1987, the Senmō Main Line was inherited by the new Hokkaido Railway Company (JR Hokkaido) as a Category-1 operator, with Japan Freight Railway (JR Freight) holding Category-2 freight running rights over it. Those freight rights were abolished on 1 April 2002, ending scheduled freight on the line and leaving it a passenger-only railway.

In its later decades the line reinvented itself around tourism. The diesel "Kushiro Shitsugen Norokko" open-window sightseeing train began running over the marsh on 24 June 1989, and a winter counterpart, the "Okhotsk Ryūhyō (drift-ice) Norokko," followed on 10 February 1990. On 8 January 2000 JR Hokkaido added the steam-hauled "SL Fuyu no Shitsugen," pulled by preserved locomotive C11 171, running between Kushiro and Shibecha (and on to Kawayu-Onsen) through the frozen wetland in winter. Local services, long worked by ageing diesel railcars, were modernised when the new H100-series DECMO cars took over the line's regular trains from 16 March 2024.

• 1887The private Kushiro Railway, a 1,067 mm gauge line, opens to carry sulphur from a mine at Atsanobori to a refinery at Shibecha; it closes about nine years later when the deposit is exhausted, but part of its formation is later reused by the Senmō line.
• 192415 November: the Abashiri Main Line is extended from Abashiri (first-generation station) to Kitahama (≈11.6 km), the first segment of what becomes the Senmō line, along the Okhotsk coast.
• 192510 November: the line is extended from Kitahama to Shari (≈25.7 km), still as the Abashiri Main Line.
• 192715 September: the Senmō Line opens from Kushiro to Shibecha (≈48.1 km), partly following the route of the defunct Kushiro Railway.
• 192915 August: the Senmō Line is extended from Shibecha to Teshikaga (≈25.3 km), climbing inland toward the Kussharo caldera.
• 192914 November: the Abashiri Main Line is extended from Shari to Sattsuru (≈19.6 km) on the northern side.
• 193020 August: the Senmō Line is extended from Teshikaga to Kawayu (now Kawayu-Onsen, ≈15.9 km).
• 193120 September: the gap between Sattsuru and Kawayu (≈22.8 km) is filled, joining the two halves and completing the through route between Kushiro and Abashiri.
• 193629 October: the through route is renamed the Senmō Main Line, the name combining characters for Kushiro (釧) and Abashiri (網).
• 19871 April: with the breakup and privatisation of Japanese National Railways, the line passes to JR Hokkaido as Category-1 operator, with JR Freight holding Category-2 freight rights.
• 198924 June: the diesel "Kushiro Shitsugen Norokko" open-air sightseeing train begins running over the Kushiro Marsh.
• 199010 February: a winter counterpart, the "Okhotsk Ryūhyō (drift-ice) Norokko," begins operation.
• 20008 January: the steam-hauled "SL Fuyu no Shitsugen," pulled by preserved locomotive C11 171, begins winter running between Kushiro and Shibecha (and on to Kawayu-Onsen) through the frozen wetland.
• 20021 April: JR Freight's Category-2 freight operation over the whole line is abolished, ending scheduled freight and leaving the line passenger-only.
• 202416 March: all regular services on the line are taken over by new H100-series (DECMO) railcars.