Rikuu East Line

The line was built in stages from both ends over the 1910s. The first section, from Kogota to Iwadeyama (24.8 km), opened on 20 April 1913 as the Rikuu Line, with new stations at Furukawa, Nakaniida and Iwadeyama. It was extended from Iwadeyama to Kawatabi (14.0 km) on 19 April 1914, and on to Naruko (6.1 km) on 18 April 1915, pushing the railhead up toward the hot springs.

Construction was meanwhile under way from the Shinjō side. On 1 November 1915 the Shinjō–Semi section (19.1 km) opened as the separate Shinjō Line, reaching Hayama-Mukaimachi (9.4 km) on 1 August 1916. The two halves were finally joined on 1 November 1917, when the Naruko–Hayama-Mukaimachi section opened to complete the through route; on the same day the Shinjō Line was absorbed into the Rikuu Line and the whole route was renamed the Rikuu East Line, with new stations at Nakayamadaira, Sakaida and Tomisawa.

The line's mountain location made it vulnerable to severe winter weather. On 3 February 1927 an avalanche struck and derailed a train near Semi, killing three people instantly, and on 13 February 1929 a Kogota-bound train was stranded by a blizzard between Nagasawa and Semi, its 134 occupants rescued only after twelve hours. Gasoline railcars began running between Kogota and Rikuzen-Furukawa on 15 May 1934. During the Second World War, on 1 December 1944, Torigoe Signal Box was opened and the Torigoe–Shinjō section (5.0 km) was singled and shared with the Ōu Main Line; this arrangement was undone on 20 December 1960, when the signal box was abolished and a separate track restored.

The postwar decades brought a steady infill of local stations and modernisation of operations. Steam haulage ended on 23 March 1974, and centralised traffic control (CTC) was introduced on 7 March 1983. With the privatisation of Japanese National Railways on 1 April 1987, freight operations between Furukawa and Shinjō were discontinued and the line was inherited by JR East as a Category 1 operator over its whole length, with Japan Freight Railway (JR Freight) taking Category 2 rights over the Kogota–Furukawa section (9.3 km); JR Freight relinquished those rights on 1 April 2002.

From the 1990s the line was rebranded around its hot springs. KiHa 110 series diesel railcars were introduced between Kogota and Naruko-Onsen on 8 December 1998, with one-man operation, and a wave of station renamings added onsen names: in 1997 Kawatabi became Kawatabi-Onsen, Naruko became Naruko-Onsen and Nakayamadaira became Nakayamadaira-Onsen, among others. On 4 December 1999 the publicly chosen nickname "Oku-no-Hosomichi Yukemuri Line" came into use, and Hayama-Mukaimachi, Semi and Uzen-Akakura were renamed Mogami, Semi-Onsen and Akakura-Onsen respectively. The Great East Japan Earthquake of 11 March 2011 suspended the whole line until 3 April, and a strong aftershock on 7 April closed it again until 16 April; in 2014 the line was incorporated into the Sendai suburban zone and Suica IC-card service began at Furukawa and Naruko-Onsen.

On 26 July 2024 heavy rain in Yamagata's Mogami region severed the Naruko-Onsen–Shinjō section, with damage at nineteen sites including a large landslide of earth and rock from the hillside beside the track; that section has remained out of service since. Restoration work on the Naruko-Onsen–Shinjō stretch began in September 2025. Today the Rikuu East Line continues to carry local KiHa 110 services across the Ōu divide, a scenic, non-electrified hot-spring route whose history spans private-era staged construction, wartime track-sharing, nationalisation and the 1987 transition to JR East.

• 191320 April: the Kogota–Iwadeyama section (24.8 km) opens as the Rikuu Line, with new stations at Furukawa, Nakaniida and Iwadeyama.
• 191419 April: the line is extended from Iwadeyama to Kawatabi (14.0 km).
• 191518 April: the Kawatabi–Naruko section (6.1 km) opens. 1 November: the Shinjō–Semi section (19.1 km) opens separately as the Shinjō Line.
• 19161 August: the Shinjō Line is extended from Semi to Hayama-Mukaimachi (9.4 km).
• 19171 November: the Naruko–Hayama-Mukaimachi section opens, completing the through route; the Shinjō Line is merged into the Rikuu Line and the whole route is renamed the Rikuu East Line, with new stations at Nakayamadaira, Sakaida and Tomisawa.
• 19273 February: an avalanche derails a southbound train near Semi, killing three people instantly.
• 193415 May: gasoline railcar operation begins between Kogota and Rikuzen-Furukawa.
• 19441 December: Torigoe Signal Box opens and the Torigoe–Shinjō section (5.0 km) is singled and shared with the Ōu Main Line (reversed in 1960).
• 197423 March: steam locomotive operation on the line ends.
• 19837 March: centralised traffic control (CTC) is introduced.
• 19871 April: with the privatisation of Japanese National Railways, Furukawa–Shinjō freight service is discontinued; JR East inherits the whole line as Category 1 operator and JR Freight takes Category 2 rights over Kogota–Furukawa (9.3 km).
• 199722 March: a wave of station renamings adds onsen names — Kawatabi becomes Kawatabi-Onsen, Naruko becomes Naruko-Onsen and Nakayamadaira becomes Nakayamadaira-Onsen, among others.
• 19988 December: KiHa 110 series diesel railcars are introduced between Kogota and Naruko-Onsen, with one-man operation.
• 19994 December: the publicly chosen nickname 'Oku-no-Hosomichi Yukemuri Line' comes into use; Hayama-Mukaimachi, Semi and Uzen-Akakura are renamed Mogami, Semi-Onsen and Akakura-Onsen.
• 20021 April: Japan Freight Railway abolishes its Category 2 rights over the Kogota–Furukawa section.
• 202426 July: heavy rain in Yamagata's Mogami region severs the Naruko-Onsen–Shinjō section, with damage at nineteen sites including a large landslide; the section remains suspended. Restoration work began in September 2025.