Sōtetsu Izumino Line

The project dates to the late 1960s. In February 1967 Sagami Railway applied for a local-railway licence for a 25.3-kilometre line from Futamata-gawa to Hiratsuka, and the licence was granted on 5 December 1968. Only the first portion was built at first: construction of the Futamata-gawa–Izumino section began on 1 March 1970. Land acquisition proved difficult, and the final parcel was not secured until the end of May 1975, delaying the work considerably.

The first section, between Futamata-gawa and Izumino, opened on 8 April 1976, three years later than originally planned, and through services with the Sōtetsu Main Line began the same day. The oil crisis had driven up costs sharply: the construction bill, originally estimated at about 5.3 billion yen, rose roughly threefold to around 14 billion yen by the time the line opened.

The line was then extended south-westwards in two further stages. Construction of the second section, from Izumino to Izumi-chūō, began on 22 August 1986, and that section opened on 4 April 1990. Work on the third section, from Izumi-chūō to Shōnandai, began in February 1994, and the line reached its present terminus at Shōnandai on 10 March 1999. Shortly before that final extension, a timetable revision on 27 February 1999 introduced Rapid services on the line for the first time.

Over the following two decades the line's express service pattern was repeatedly reworked. Limited Express services were introduced in a revision on 27 April 2014, then abolished in a revision on 30 November 2019, which instead created Commuter Limited Express and Commuter Express services. Across these changes Rapid and other express categories were variously suspended and restored as Sagami Railway adjusted its daytime and rush-hour patterns. The line was also caught up in two rare industrial actions: strikes on 26 June 2009 and 20 March 2014 each suspended services across the whole line from the first train until 7 a.m.

The most significant change came in 2023, when the line was incorporated into a new cross-Tokyo through-running network. The route's name was changed to "Sōtetsu Izumino Line" that year, having previously been known simply as the Izumino Line. On 18 March 2023 the opening of the Sōtetsu Shin-yokohama Line (Hazawa Yokohama-kokudai–Shin-yokohama) and the connecting Tōkyū Shin-yokohama Line inaugurated direct services from Shōnandai via the Sōtetsu Main Line and these new links onto the Tōkyū Tōyoko Line, the Tokyo Metro Fukutoshin Line and the Tōbu Tōjō Line, and a Limited Express running through to the Tōkyū lines was reinstated.

Infrastructure was modernised in parallel. Platform-edge doors entered service at platforms 1 and 2 of Futamata-gawa Station on 24 January 2021 and were then rolled out station by station; with their introduction at Yumegaoka Station on 2 July 2023, every station on the Sōtetsu Izumino Line had been fitted with platform doors. Today the line functions as a suburban commuter route knitting the south-western Yokohama and Fujisawa suburbs into both the Sōtetsu network and, through it, the wider Tokyo rail system.

• 1967February: Sagami Railway applies for a local-railway licence for the 25.3 km Futamata-gawa–Hiratsuka line.
• 19685 December: the local-railway licence for the Futamata-gawa–Hiratsuka line is granted.
• 19701 March: construction of the first-stage section, Futamata-gawa–Izumino, begins; land acquisition is not completed until the end of May 1975.
• 19768 April: the Futamata-gawa–Izumino section opens (three years behind the original plan) and through running with the Sōtetsu Main Line begins; oil-crisis cost rises pushed construction from ~5.3 billion to ~14 billion yen.
• 198622 August: construction of the second-stage section, Izumino–Izumi-chūō, begins.
• 19904 April: the Izumino–Izumi-chūō section opens.
• 1994February: construction of the third-stage section, Izumi-chūō–Shōnandai, begins.
• 199927 February: a timetable revision introduces Rapid services on the line for the first time.
• 199910 March: the Izumi-chūō–Shōnandai section opens, completing the line to its present terminus at Shōnandai.
• 200926 June: a strike — the first by a major Kantō private railway in roughly 20 years — suspends services across the whole line from the first train until 7 a.m.
• 201420 March: a strike again suspends services across the whole line from the first train until 7 a.m.
• 201427 April: a timetable revision introduces Limited Express services on the line; daytime Rapid services are suspended.
• 201930 November: a timetable revision abolishes the Limited Express and creates new Commuter Limited Express and Commuter Express services.
• 202124 January: platform-edge doors enter service at platforms 1 and 2 of Futamata-gawa Station, beginning a station-by-station rollout.
• 202318 March: the line is renamed the "Sōtetsu Izumino Line" (formerly the Izumino Line); with the opening of the Sōtetsu Shin-yokohama Line and the Tōkyū Shin-yokohama Line, through services begin from Shōnandai onto the Tōkyū Tōyoko, Tokyo Metro Fukutoshin and Tōbu Tōjō lines, and a Tōkyū-bound Limited Express is reinstated.
• 20232 July: platform-edge doors enter service at Yumegaoka Station, completing platform-door installation at every station on the Sōtetsu Izumino Line.