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Sotobo Line

外房線

The Sotobō Line is a railway line in Chiba Prefecture, Japan, operated by the East Japan Railway Company (JR East).

Running beside the Pacific Ocean along the eastern, or outer, side of the Bōsō Peninsula, it connects Chiba Station in Chiba to Awa-Kamogawa Station in Kamogawa, crossing the peninsula from Chiba before turning south down its Pacific coast through Ōamishirasato, Mobara, Chōsei, Ichinomiya, Isumi, Onjuku and Katsuura. The line is 93.3 kilometres long, has 27 stations, is built to 1,067 mm narrow gauge, and is connected to the Uchibō Line at both ends: at Soga, where the Uchibō Line branches off, and again at the southern terminus of Awa-Kamogawa.

Route of the Sotobo Line · Chiba · Borders: MLIT
A 209 series 2100 subseries train entering Hon-Chiba Station on the Sotobo Line.
A 209 series 2100 subseries train entering Hon-Chiba Station on the Sotobo Line. · Jet-0 · CC BY-SA 3.0 · Wikimedia Commons

History

The line's origins lie with a private company, the Boso Railway (Bōsō Tetsudō), conceived as a railway encircling the Pacific side of the Bōsō Peninsula. It opened its first section, between Soga and Ōami, in January 1896, and extended to Chiba (connecting there with the Sōbu Railway) in February 1896. The line was pushed on to Ichinomiya — today's Kazusa-Ichinomiya — in 1897 and to Ōhara in 1899. In 1907 the Boso Railway was nationalised under the Railway Nationalization Act and absorbed into the government railways; when names were given to the national railway lines in 1909, the Chiba–Ōhara section became the Boso Line. Extensions then continued down the coast: to Katsuura in 1913, to Kazusa-Okitsu in 1927, and finally to Awa-Kamogawa in 1929, at which point the route was complete and was joined to the Hōjō Line (the forerunner of today's Uchibō Line). The naming of the line shifted with this growth: in 1933 the Chiba–Ōami–Awa-Kamogawa portion was separated out as the Boso East Line (Bōsō-tōsen), and that name lasted until 1972, when the line was renamed the Sotobō Line.

The railway was rebuilt and modernised in stages through the twentieth century. At Toke, where the old tunnel was narrow and ageing, the line's loading gauge was improved. Track-doubling was carried out piecemeal over decades: the Chiba–Soga section was double-tracked between 1960 and 1963, the last stage coinciding with the 1963 relocation of Chiba Station, which removed the reversing (switchback) movement there. The relocation of Ōami Station in 1972 removed a second switchback, after which trains could run from the Tokyo direction towards Ōhara without changing direction. Double track was extended to Nagata between 1972 and 1974, and the line as far as Kazusa-Ichinomiya was progressively doubled between 1980 and 1986; further double-track sections followed, with the Onjuku–Katsuura stretch doubled in 1995 and the Torami–Chōjamachi stretch in 1996. Even so, the line is not double-tracked throughout: single-track sections remain south of Kazusa-Ichinomiya, while the busier section north of Kazusa-Ichinomiya is double track. Centralised Traffic Control (CTC) was commissioned over the Soga–Awa-Kamogawa section in the 1970s. The whole line is electrified at 1,500 V DC overhead; the Chiba–Soga section was electrified in July 1968. The 183-series limited express "Wakashio" began operating in July 1972, on the day the line was renamed the Sotobō Line. Freight services were withdrawn section by section from most of the line between 1982 and 1987.

Today JR East operates local services together with Rapid and limited express services. The limited express "Wakashio" runs from Tokyo via the Keiyō Line and Soga to Awa-Kamogawa; a weekend "Shinjuku Wakashio" also operates. Rapid services run through from the Sōbu Line (Rapid) and Yokosuka Line and from the Keiyō Line. The northern, double-tracked Chiba–Kazusa-Ichinomiya section carries comparatively frequent service, including through trains from the Tōkyō direction and all-stations services from the Tōgane Line, whereas service thins out further south, where the line has single-track sections. Commuter Rapid services that once ran through to and from the Keiyō Line were discontinued with the timetable revision of 16 March 2024. Among the rail lines of the Bōsō area, the Sotobō Line is the busiest, ranking first in passenger-kilometres, average number of passengers carried, and passenger revenue.

Sotobo Line limited express Wakashio: an E257 series and a 255 series side by side at Oami Station. View full size (opens in a new tab)
Sotobo Line limited express Wakashio: an E257 series and a 255 series side by side at Oami Station.Iozdff · CC BY-SA 4.0 · Wikimedia Commons

On 8 May 2020, at about 3:53 pm, the front carriage of a 209-series local service from Awa-Kamogawa to Chiba derailed between Awa-Kamogawa and Awa-Amatsu stations; 16 passengers and three crew were aboard, and one passenger was slightly injured. The Japan Transport Safety Board concluded that the leading bogie had probably ridden up on several pieces of ballast lying near the top of the rails in a level crossing; stones may have been placed there, but the board could not establish the details.

Stations on this line

27 stations tracked on the map, listed alphabetically. Select any to read its page.

Timeline
  • 1896January: the Boso Railway opens its first section, Soga–Ōami. In February the line is extended to Chiba, connecting with the Sōbu Railway.
  • 1897The line is extended to Ichinomiya (present-day Kazusa-Ichinomiya).
  • 1899The line is extended to Ōhara.
  • 19071 September: the Boso Railway is nationalised under the Railway Nationalization Act and absorbed into the government railways; its Chiba–Ōhara line is named the Boso Line on 12 October 1909.
  • 191320 June: the line is extended from Ōhara to Katsuura.
  • 19271 April: the line is extended from Katsuura to Kazusa-Okitsu.
  • 192915 April: the line is extended from Kazusa-Okitsu to Awa-Kamogawa, completing the route and joining the Hōjō Line (forerunner of the Uchibō Line).
  • 19331 April: the Chiba–Ōami–Awa-Kamogawa section is separated out as the Boso East Line (Bōsō-tōsen).
  • 1954Works at Toke, where the old tunnel was narrow and ageing, are completed, improving the line's loading gauge (work begun 1952).
  • 196328 April: Chiba Station is relocated, eliminating the switchback there; the Chiba–Soga section is double-tracked (completed 1960–1963).
  • 196813 July: the Chiba–Soga section is electrified (1,500 V DC).
  • 197215 July: the line is renamed the Sotobō Line and the 183-series limited express 'Wakashio' begins operating.
  • 1986The line to Kazusa-Ichinomiya is fully double-tracked (progressive doubling 1980–1986).
  • 19871 April: JNR is privatised; the line transfers to JR East. JR Freight becomes a Type II operator on the Chiba–Soga section.
  • 199526 November: the Onjuku–Katsuura section is double-tracked.
  • 199617 November: the Torami–Chōjamachi section is double-tracked.
  • 200416 October: E257-500 series EMUs are introduced on the 'Wakashio' limited express.
  • 20208 May, about 3:53 pm: the front carriage of a 209-series local service derails between Awa-Kamogawa and Awa-Amatsu, with 16 passengers and three crew aboard; one passenger is slightly injured. The Japan Transport Safety Board later finds that the train probably ran onto several pieces of ballast lying near the top of the rails at a level crossing; stones may have been placed there, but the details could not be established.
  • 202113 March: E131 series EMUs are introduced on Kazusa-Ichinomiya–Awa-Kamogawa with one-man operation and through service to the Uchibō Line.
  • 202416 March: Commuter Rapid services through to/from the Keiyō Line are discontinued.

Sources

Last updated 16 October 2026.

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