Suigun Line

Although the line officially ends at Asaka-Nagamori, no trains reverse there: every Fukushima-side service continues over a short stretch of the Tōhoku Main Line to Kōriyama Station, so in practice it runs Mito to Kōriyama. The route threads the valley of the Kuji River through the Okukuji district of northern Ibaraki — an area of clear water and gorges, with the well-known Fukuroda Falls near Fukuroda Station — before climbing into the hills of southern Fukushima. As of the timetable revised on 16 March 2024 only five round trips a day run the full Mito–Kōriyama length; the Mito–Kami-Sugaya section, used by both main-line and branch trains, is the busiest with one to two trains an hour, while the Hitachi-Daigo–Kōriyama section is the thinnest, with gaps of several hours during the day.

The line's origins are private. The Ota Horse Tramway was authorised in 1892 to build from Mito towards Ōta, but was reorganised as the steam-powered Ota Railway, which opened the Mito–Kuji-gawa section on 16 November 1897 and extended to Ōta (now Hitachi-Ōta) in 1899. Burdened by the cost of bridging the Kuji River and by management troubles, the Ota Railway transferred its undertaking on 21 October 1901 to a newly formed Mito Railway (the second of that name), set up after the Fifteenth Bank seized the assets over the Ota Railway's defaults. It was the Mito Railway that opened the Kami-Sugaya–Hitachi-Ōmiya section in 1918, the change that turned the original Kami-Sugaya–Hitachi-Ōta segment into a branch.

The main line was pushed northward in stages. The Mito Railway reached Hitachi-Daigo on 10 March 1927, and on 1 December that year it was nationalised, the combined Mito–Hitachi-Daigo and Kami-Sugaya–Hitachi-Ōta route being designated the Suigun Line. From the Fukushima end, Japanese Government Railways built a separate line — the two halves renamed the Suigun South and Suigun North lines in 1929 — working south from Asaka-Nagamori and north from Hitachi-Daigo through the 1930s. The final gap, Iwaki-Tanakura to Kawahigashi, closed on 4 December 1934, completing a continuous Mito–Asaka-Nagamori line, and the two halves were merged back as the Suigun Line.

Diesel railcar operation, suited to the unelectrified line, was introduced from the 1930s, and by 1959 all passenger trains had been converted to diesel power. Freight ended on the Kami-Sugaya–Hitachi-Ōta branch in 1982 and over the Mito–Asaka-Nagamori main line in 1987, and centralised traffic control was commissioned over the whole line on 1 June 1983. With the privatisation of Japanese National Railways on 1 April 1987, the Suigun Line passed to JR East. It is operated end to end with one-person (driver-only) operation, and its present rolling stock is the KiHa E130 series diesel multiple unit, which entered service on 19 January 2007 and fully replaced the earlier KiHa 110 series by that September.

The Suigun Line has twice been severed by natural disaster in recent years. The 2011 Great East Japan Earthquake damaged track and structures and closed the whole line, which reopened in stages that April. Far more serious was Typhoon Hagibis: on 13 October 2019 its torrential rain brought down the line's sixth Kuji River bridge between Fukuroda and Hitachi-Daigo and washed away the second Yashirogawa bridge between Iwaki-Asakawa and Satoshiraishi, closing the entire line. Service was restored section by section with substitute buses bridging the gaps, but repairs to the collapsed Kuji River bridge took far longer: the line did not return to through operation until the Fukuroda–Hitachi-Daigo section reopened on 27 March 2021, more than seventeen months after the typhoon.

• 1892July: the Ota Horse Tramway is authorised to build a line from Mito towards Ōta (present-day Hitachiōta); it is later reorganised as the steam-powered Ota Railway.
• 189716 November: the Ota Railway opens its first section, Mito to Kuji-gawa; the line's oldest segment.
• 1899The Ota Railway extends from Kuji-gawa to Ōta (now Hitachi-Ōta), reaching the present branch terminus.
• 190121 October: after the Ota Railway's bankruptcy, its undertaking is transferred to the newly formed Mito Railway (2nd), set up by creditor banks including the Fifteenth Bank.
• 1918The Mito Railway opens the Kami-Sugaya to Hitachi-Ōmiya section, the change that turns the original Kami-Sugaya–Hitachi-Ōta segment into a branch.
• 192710 March: the line reaches Hitachi-Daigo. 1 December: the Mito Railway is nationalised; the Mito–Hitachi-Daigo and Kami-Sugaya–Hitachi-Ōta route is named the Suigun Line.
• 1929Japanese Government Railways opens the Asaka-Nagamori-side line (the Suigun North Line) from Asaka-Nagamori; the existing line is renamed the Suigun South Line.
• 19344 December: the final gap, Iwaki-Tanakura to Kawahigashi, opens, completing the Mito–Asaka-Nagamori line; the two halves are merged back as the Suigun Line.
• 19591 November: all passenger trains on the line are converted to diesel traction.
• 19821 October: freight operation on the Kami-Sugaya–Hitachi-Ōta branch ends.
• 19831 June: centralised traffic control (CTC) is commissioned over the entire line.
• 19871 April: freight operation on the Mito–Asaka-Nagamori main line ends; with the privatisation of JNR, the Suigun Line passes to JR East.
• 200719 January: KiHa E130 series DMUs enter service; they replace the KiHa 110 series by September the same year.
• 201111 March: the Great East Japan Earthquake damages track and structures and closes the whole line, which reopens in stages in April.
• 201913 October: Typhoon Hagibis brings down the sixth Kuji River bridge (Fukuroda–Hitachi-Daigo) and washes away the second Yashirogawa bridge (Iwaki-Asakawa–Satoshiraishi), closing the whole line.
• 202127 March: the Fukuroda–Hitachi-Daigo section reopens after bridge repairs, restoring through operation over the whole line more than seventeen months after Typhoon Hagibis.