Isumi Line

Construction of the Kihara Line (木原線) was authorised under the 1922 revised Railway Construction Act, but no firm date was set. A Chiba Prefectural human-powered tramway, and its successor the Isumi Tramway (夷隅軌道), had been operating between Ōhara and Ōtaki, but were running at a loss the local economy could not sustain. Local residents lobbied the Railway Ministry hard, since a state-built railway would shift construction and operating costs to the government, and construction of the Kihara Line was approved in 1925. The earlier tramway was abolished once the new line opened. The name "Kihara" was formed from the 木 of Kisarazu and the 原 of Ōhara: the line was planned, under Appendix Clause 48 of the revised Railway Construction Act, to link Ōhara with Kisarazu by connecting to the Kururi Line, but the section onward from Kazusa-Nakano toward Kazusa-Kameyama was never built.

The first section of the Kihara Line, the 15.9 km from Ōhara to Ōtaki, opened on 1 April 1930, bringing into use the stations at Kazusa-Azuma, Kuniyoshi and Kazusa-Nakagawa. It was extended a further 6.4 km from Ōtaki to Fusamoto on 25 August 1933, and a final 4.6 km from Fusamoto to Kazusa-Nakano on 26 August 1934, completing the Ōhara–Kazusa-Nakano route at a length of 26.9 km. In its first years steam locomotives of JNR Classes C10 and C12 hauled a few passenger carriages; from 1934 KiHa 40000 gasoline railcars were introduced to increase the number of services, and during the Pacific War these were converted to run on locally produced natural gas to cope with fuel shortages until 1944.

After the war services reverted to C12-hauled carriages, and in 1954 the KiHa 10000 railbus was introduced to cut costs and add more frequent services with smaller cars; its 56-seat capacity proved too small, however, and problems such as the lightweight cars failing to trigger level-crossing detectors ended its use after seven years. The line ran through a sparsely populated district and never carried heavy traffic. On 4 September 1968 it was named by a JNR advisory committee as one of the loss-making local lines (the "83 deficit lines") recommended for closure, and in the summer of 1970 heavy rain washed out the roadbed and severed the line, suspending services from 1 July until full restoration on 1 October. Freight operations were discontinued on 1 October 1974.

Under the 1980 JNR Reconstruction Act the Kihara Line was designated a first-round Specified Local Line on 18 September 1981, marking it for conversion or closure. With the breakup and privatisation of JNR on 1 April 1987 it passed to the East Japan Railway Company (JR East), and on 24 March 1988 the JR Kihara Line was abolished and reborn as the Isumi Railway Isumi Line, operated by the newly formed third-sector Isumi Railway Company — the last of the first-round Specified Local Lines to be converted. At the same time the Ōhara–Nishi-Ōhara section was re-measured from 1.8 km to 1.7 km, reducing the line's operating length from 26.9 km to its present 26.8 km.

Burdened by chronic deficits and facing possible abolition, the Isumi Railway pursued a series of revenue and ridership measures from the late 2000s. It opened a new station at Shiromigaoka in 2008, sold naming rights to stations to local businesses, and launched Moomin-themed trains on 1 October 2009, drawing tourists to the rural line. From 29 April 2011 a "sightseeing express" ran mainly at weekends using KiHa 52 125, a diesel car acquired from JR West and repainted in JNR red-and-cream livery; from 9 March 2013 it was joined by KiHa 28 2346, another former JR West car, and one of the pair was fitted out as a "Restaurant KiHa" serving Italian cuisine. After its finances recovered, the line's continuation was confirmed on 6 August 2010.

The Moomin trains ended at the close of March 2019, and the heritage diesel cars were gradually withdrawn — KiHa 28 2346 ending revenue service on 8 February 2023, and the sightseeing express ceasing regular operation in the 16 March 2024 timetable revision. On 4 October 2024 a two-car train derailed near a level crossing between Kuniyoshi and Kazusa-Nakagawa; the whole line was suspended and replaced by substitute buses. The railway attributed the derailment to poor track maintenance, and as of 2025 services on the Ōhara–Ōtaki section were planned to resume in the autumn of 2027.

• 191215 December: a 609 mm-gauge human-powered tramway opened by the Chiba Prefectural government connects Ōhara and Ōtaki, the predecessor of the future railway corridor.
• 1922Construction of the Kihara Line is authorised under the revised Railway Construction Act (Appendix Clause 48), as a planned Ōhara–Kisarazu link via the Kururi Line; no date is fixed.
• 1925Construction of the Kihara Line is approved; the existing loss-making Isumi Tramway is later abolished when the new line opens.
• 19301 April: the first section of the Kihara Line, Ōhara–Ōtaki (15.9 km), opens; Kazusa-Azuma, Kuniyoshi and Kazusa-Nakagawa stations open.
• 193325 August: the line is extended from Ōtaki to Fusamoto (6.4 km).
• 193426 August: the final section, Fusamoto–Kazusa-Nakano (4.6 km), opens, completing the 26.9 km Ōhara–Kazusa-Nakano line; diesel-car operation begins on 15 September.
• 19371 February: Higashi-Fusamoto and Nishihata stations open.
• 196020 June: Nishi-Ōhara, Nittano, Koyamatsu and Kugahara stations open.
• 19684 September: the Kihara Line is named by a JNR advisory committee as one of the loss-making local lines (the '83 deficit lines') recommended for closure.
• 19701 July: heavy rain washes out the roadbed and severs the line; services are suspended until full restoration on 1 October.
• 19741 October: freight operations on the line are discontinued.
• 198118 September: under the JNR Reconstruction Act the Kihara Line is designated a first-round Specified Local Line, marking it for conversion or closure.
• 19871 April: with the breakup and privatisation of JNR, the line passes to the East Japan Railway Company (JR East).
• 198824 March: the JR Kihara Line is abolished and reopens as the Isumi Railway Isumi Line under the new third-sector Isumi Railway Company — the last first-round Specified Local Line to be converted; re-measurement of the Ōhara–Nishi-Ōhara section cuts the line from 26.9 km to 26.8 km.
• 20089 August: a new station, Shiromigaoka, opens as part of the railway's revival efforts.
• 20091 October: Moomin-themed trains begin running, drawing tourists to the rural line.
• 201129 April: a 'sightseeing express' using ex-JR West diesel car KiHa 52 125, repainted in JNR red-and-cream livery, begins running mainly at weekends between Ōtaki and Ōhara.
• 2019End of March: the Moomin trains cease operation, with a commemorative event held at Ōtaki Station.
• 202416 March: the sightseeing express ends regular operation; on 4 October a two-car train derails near a level crossing between Kuniyoshi and Kazusa-Nakagawa, suspending the whole line and prompting substitute bus services.
• 2027Autumn (planned): services on the Ōhara–Ōtaki section are scheduled to resume, at an estimated restoration cost of around 1.4 billion yen.