Ōu Main Line

The Japanese national government built the line in two halves that grew toward each other. The Ōu North Line was begun at Aomori in 1894 and the Ōu South Line at Fukushima in 1899, the two being joined in 1905. The northern half opened in stages from the Aomori end: Aomori–Hirosaki on 1 December 1894 (worked at first by three round trips a day), Hirosaki–Ikarigaseki in 1895, then on through Ōdate, Noshiro, Akita and Ōmagari to Yokote on 15 June 1905. The southern half advanced from Fukushima: Fukushima–Yonezawa on 15 May 1899, then up through Yamagata, Shinjō and Yuzawa, with Yuzawa–Yokote opening on 14 September 1905 to complete the through Fukushima–Aomori connection. The government railway's line-naming list of 12 October 1909 formally designated the whole route the Ōu Main Line.

The line's defining engineering challenge is the mountain country of its southern half. Out of Fukushima the railway climbs the Itaya Pass on the Fukushima–Yamagata border, where the gradient reaches 38.0 per mille (the steepest on the line, between Tōge and Ōsawa) and small mountain stations — Itaya, Tōge and Ōsawa among them — were built as switchback (reversing) stations clear of the climbing main track. Itaya was rebuilt from a reversing into a through station during the gauge-conversion work in 1990. Lower down, the Niwasaka–Akaiwa section proved geologically unstable: an original tunnel collapsed in 1910, with a two-tunnel realignment opening the next year, and a 1948 derailment that killed three crewmen prompted a further realignment when the section was double-tracked in 1968. Double-tracking of various stretches proceeded from 1963 onward.

Electrification came piecemeal and changed standard midway. Fukushima–Yonezawa was first electrified at 1,500 V DC in 1949, and Uzen-Chitose–Yamagata — with the connecting Senzan Line — at the same voltage in 1960. Trials on the Senzan Line led the railway to adopt 20 kV AC for all later work, and the earlier DC sections were converted when Yonezawa–Yamagata was electrified in 1968. Aomori–Akita followed at 20 kV AC in 1971 and Akita–Uzen-Chitose in 1975. Much later, after a July 2024 rainstorm forced a months-long closure between Shinjō and Innai, that section was rebuilt but its overhead wiring decommissioned, and trains resumed there without electrification on 25 April 2025 — only the second such de-electrification in the JR East area.

The modern transformation of the line was the "mini-Shinkansen" conversion of two sections to standard gauge. Rather than build a separate high-speed 1,435 mm alignment, the mini-Shinkansen approach re-gauges an existing narrow-gauge main line so that purpose-built Shinkansen trains can run directly onto it, leaving the loading gauge and (here) the voltage unchanged. After the 1987 privatisation of Japanese National Railways, JR East chose to convert the Fukushima–Yamagata section of the 1,067 mm line to 1,435 mm so that trains from Yamagata could continue onto the Tōhoku Shinkansen through to Tokyo. The Fukushima–Shinjō tracks were re-gauged between 1988 and 1992, and the Yamagata Shinkansen opened on 1 July 1992 as the Tsubasa; though top speed on the conventional section is only 130 km/h, ending the change of trains at Fukushima cut journey times.

The success of the Yamagata project led to a second conversion. Ōmagari–Akita was re-gauged for the opening of the Akita Shinkansen on 22 March 1997, running as the Komachi, and the Yamagata Shinkansen was extended on to Shinjō in 1999. As these conversions were on mostly single-track railway, parallel 1,435 mm and 1,067 mm tracks were created between Ōmagari and Akita and between Yamagata and Uzen-Chitose, with a dual-gauge (three-rail) section between Jingūji and Mineyoshikawa, so standard-gauge Shinkansen and surviving narrow-gauge local trains could both run and Shinkansen could pass on the single-track line.

These changes split the through line into four operationally distinct sections, and because their gauges differ essentially no train runs across more than one (apart from an Akita–Shinjō connection). The standard-gauge Fukushima–Shinjō section (148.6 km) is shared with the Yamagata Shinkansen and bears the Yamagata Line nickname; the narrow-gauge Shinjō–Ōmagari section (98.4 km) into Akita carries lightly used local trains; Ōmagari–Akita (51.7 km) is shared with the Akita Shinkansen; and the narrow-gauge Akita–Aomori section (185.8 km) forms part of the Nihonkai Jūkan (Sea-of-Japan trans-Japan) corridor used by express and freight trains. On the converted sections, local trains are standard-gauge 701-5000 series. The line passed to JR East at the 1 April 1987 privatisation, with Japan Freight Railway working freight over much of the route.

• 18941 December: the Japanese government opens the first stretch of the Ōu North Line, Aomori–Hirosaki, worked at first by three round trips a day.
• 189521 October: the Hirosaki–Ikarigaseki section opens.
• 189915 May: construction of the southern half begins with the opening of Fukushima–Yonezawa; in the same year the northern half is extended Ikarigaseki–Shirasawa (21 June) and Shirasawa–Ōdate (15 November).
• 190111 April: the Yonezawa–Yamagata section opens, reaching the city of Yamagata on the southern half.
• 190515 June: the northern half reaches Yokote (Ōmagari–Yokote). 14 September: the Yuzawa–Yokote section opens, completing the through Fukushima–Aomori connection.
• 190912 October: under the government railway's line-naming list, the whole Fukushima–Aomori route is formally designated the Ōu Main Line (奥羽本線).
• 1910One of the original tunnels on the geologically unstable Niwasaka–Akaiwa section collapses; a realignment with two new tunnels opens the following year.
• 1948A derailment on the Niwasaka–Akaiwa section kills three crewmen; geological instability is suspected, and a further realignment accompanies double-tracking of the section in 1968.
• 1949The Fukushima–Yonezawa section is electrified at 1,500 V DC, the line's first electrification.
• 1968The Yonezawa–Yamagata section is electrified; the earlier 1,500 V DC sections are converted to the 20 kV AC standard adopted after Senzan Line trials.
• 1971The Aomori–Akita section is electrified at 20 kV AC; the Akita–Uzen-Chitose section follows in 1975.
• 19871 April: at the privatisation of Japanese National Railways the line passes to JR East, with Japan Freight Railway as freight operator over much of the route.
• 1990Itaya station, originally a switchback (reversing) station on the Itaya Pass, is rebuilt as a through station in conjunction with the gauge-conversion work.
• 19921 July: after the Fukushima–Shinjō tracks are re-gauged from 1,067 mm to 1,435 mm (1988–1992), the Yamagata Shinkansen opens over the Fukushima–Yamagata section under the Tsubasa service name.
• 199722 March: the Akita Shinkansen opens after the Ōmagari–Akita section is re-gauged, with a dual-gauge (three-rail) segment between Jingūji and Mineyoshikawa, running under the Komachi service name.
• 1999The Yamagata Shinkansen is extended from Yamagata on to Shinjō, lengthening the standard-gauge mini-Shinkansen section.
• 202525 April: after a July 2024 rainstorm forced a months-long closure, the Shinjō–Innai section reopens rebuilt but de-electrified — only the second such de-electrification in the JR East area.