The Sotobo Line (外房線) is a main line run by East Japan Railway Company (JR East). It runs from Chiba Station in Chiba City, through Mobara Station in Mobara, to Awa-Kamogawa Station in Kamogawa, Chiba.
History
The line started as the Boso Railway (房総鉄道). In 1893 the Boso Tramway Company (房総鉄道馬車会社) was granted a railway licence for the route between Soga and Oami. On 20 January 1896 the Soga to Oami section opened, and on 25 February 1896 the Chiba to Soga section opened. In February the line connected at Chiba with the Sobu Railway (総武鉄道). The line reached Ichinomiya on 17 April 1897 and Ohara on 13 December 1899. On 1 September 1907 the Boso Railway was bought by the state under the Railway Nationalization Act (鉄道国有法) and became part of the national railway. On 12 October 1909 the Chiba to Ohara section was named the Boso Line (房総線).
The line was extended east in stages. Katsuura opened on 20 June 1913. Tojimi station opened on 15 December 1925. The Hokujo Line (北条線), which had reached Awa-Kamogawa in 1925, was merged into the Boso Line in 1929. The section from Katsuura to Kazusa-Okitsu opened on 1 April 1927. The last section to Awa-Kamogawa opened on 15 April 1929, which completed the line. The Hokujo Line is now the Uchibo Line (内房線). On 1 April 1933 the line was split. Chiba to Oami to Awa-Kamogawa became the Boso East Line (房総東線). Diesel railcars began running between Chiba and Hon-Chiba on 15 September 1933, and between Hon-Chiba and Oami on 1 July 1935. Daigan-ji station opened between Soga and Honda on 15 January 1938 and closed on 10 August 1941.
In 1952 work began to remove the old Tsuke Tunnel, which was narrow and aging. Part of the old tunnel was left in place, and the work was completed in 1954. Kamatori station opened on 15 June 1952. On 1 October 1954 almost all trains became diesel railcars, and a timetable with one-hour intervals was introduced. Shin-Mobara station opened on 15 September 1955, and Nagata station on 20 March 1959. From 1960 the Hon-Chiba to Soga section was doubled and given automatic signals. On 28 April 1963 Chiba station was moved toward Nishi-Chiba, which removed the switchback at Chiba. Chiba to Hon-Chiba was doubled at the same time. On 27 May 1972 Oami station was moved, which removed its switchback. The two moves let trains run from Tokyo toward Ohara without changing direction, and made the line faster. However, after the Oami switchback was removed, trains that circled the peninsula could not be scheduled because of the automatic train control (ATC) on the Sobu Main Line. On 1 February 1965 ATS-S was introduced across the whole line. Chiba to Soga was electrified on 13 July 1968. On 20 August 1969 a farewell run pulled by steam locomotive C57 105 marked the end of regular steam-hauled passenger trains. Namegawa Island station opened as a temporary station on 2 July 1970. On 15 July 1972 the line was renamed the Sotobo Line (外房線). Soga to Awa-Kamogawa was electrified, and the 183 series began the Wakashio (わかしお) limited express.
Double track was added in stages. Honda to Tuke was doubled on 1 July 1973. Kamatori to Honda was doubled on 25 October 1974, and Soga to Kamatori on 12 December 1974. On 17 March 1978 a temporary service began to carry aviation fuel to New Tokyo International Airport (now Narita International Airport), because the full pipeline was not yet built. The Chiba to Soga section was used for it. Yatsumi to Kazusa-Ichinomiya was doubled on 11 September 1980. Freight service between Ohara and Awa-Kamogawa ended on 15 November 1982. Nagata to Honno was doubled on 25 February 1983, and freight service between Shin-Mobara and Ohara ended on 1 February 1984. Honno to Shin-Mobara was doubled on 1 March 1985. On 27 October 1986 Shin-Mobara to Yatsumi was doubled, and Mobara station was rebuilt as an elevated station with one island platform and two tracks. On 1 April 1987 the national railway was split up and privatised. East Japan Railway Company (JR East) took over the line. Freight service from Soga to Oami ended, and Japan Freight Railway Company (JR Freight) became the freight operator on the Chiba to Soga and Oami to Shin-Mobara sections. Namegawa Island station became a permanent station. On 10 December 1987 Mobara station became an elevated station with two island platforms and four tracks.
Double track and faster running continued. On 26 November 1995 Oshuku to Katsuura was doubled, and from 1 December 1995 the top speed between Soga and Katsuura was raised to 120 km/h. On 17 November 1996 Tojimi to Chojamachi was doubled. On 22 March 1997 smoking was banned on local trains from Ohara to Awa-Kamogawa. On 8 December 1998 Kamatori became a rapid train stop, and on 4 December 1999 Tuke did too. JR Freight's operation between Oami and Shin-Mobara ended on 31 March 1999. ATS-P began between Chiba and Soga on 6 February 2000, and between Soga and Kazusa-Ichinomiya on 17 August 2000. On 18 November 2001 the Suica IC card service began between Chiba and Mobara, which was then part of the Tokyo suburban area. On 16 October 2004 E257 500 series trains were introduced on the Wakashio. Mobara to Ohara joined the Tokyo suburban area, and Suica service started there too. On 14 March 2009 Ohara to Awa-Kamogawa joined the Tokyo suburban area, with Suica service. On 1 October 2009 the 209 series began to enter service. On 20 December 2009 ATS-P began between Kazusa-Ichinomiya and Awa-Kamogawa. On 10 February 2010 the CTC and PRC control system was renewed. On 4 December 2010 Hon-Chiba became a rapid train stop. On 1 September 2011 the 113 series ended its regular service on the line. Its farewell run on 24 September 2011 ended a run on this line that began in 1969 and lasted about 42 years.
In November 2015 a temporary rapid train called Osanpo Kawagoe (おさんぽ川越号) began running from Mobara to Kawagoe, and it later ran occasionally. On 22 August 2016 Typhoon No. 9 caused damage, mostly south of Kazusa-Ichinomiya. Trains were suspended from midday on 22 August to the night of 23 August, and the Wakashio was also suspended for those days. On 23 October 2017 Typhoon No. 21 suspended trains between Katsuura and Awa-Kamogawa until 10:42, and between Tuke and Honda until about 15:30, because of fallen trees and pantograph damage. On 13 January 2018 a cycle train called B.B.BASE Gaiboso, run by BOSO BICYCLE BASE, began running from Ryogoku to Katsuura. On 8 May 2020 a local train from Awa-Kamogawa to Chiba derailed between Awa-Tenno and Awa-Kamogawa at about 15:55. Service was suspended for the rest of the day and resumed on 9 May. On 19 November 2020 tsunami evacuation guidance signs were completed. On 13 March 2021 E131 series trains began running between Kazusa-Ichinomiya and Awa-Kamogawa, with one-man operation. Through running with the Uchibo Line (内房線) between Awa-Kamogawa and Kisarazu also began.
On 8 September 2023 Typhoon No. 13 brought heavy rain, and a slope beside the up track between Tuke and Oami slipped. Trains were suspended between Honda and Awa-Kamogawa. Oami to Awa-Kamogawa service resumed on 11 September, and Honda to Oami on 12 September 2023. On 1 July 2026 regional branch offices were replaced by business divisions. Chiba to Hon-Chiba came under the Chiba Business Division, and Soga to Awa-Kamogawa came under the Boso Business Division. On 13 August 2026 heavy rain across Chiba Prefecture suspended trains from Chiba to Awa-Kamogawa from the afternoon until the next day. Replacement buses ran from 17 August between Honda and Tuke, between Oami and Honno, and between Soga and Mobara. Oami to Honno resumed from the first train on 18 August, and Honda to Oami on 23 August, when the whole line was running again. On 21 September 2026 heavy rain from Typhoon No. 25 suspended the whole line, and it resumed on 24 September.
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Written by an AI from the line's history on Japanese Wikipedia, nothing else, and checked by a program for dates and numbers that are not in those facts. It can still be wrong. Adapted from Japanese Wikipedia, CC BY-SA 4.0; this text is shared under the same licence.