Sōtetsu Shin-Yokohama Line

The line originated as part of the Kanagawa East-West Line plan, a corridor identified in the old Transport Policy Council's reports No. 7 and No. 18. In the 2000s Sōtetsu branded the project the "Metropolitan Through Service Project," intended to link the Sōtetsu Main Line with JR East's Shōnan-Shinjuku Line and with the Tōkyū Tōyoko and Meguro lines to create an access route into the centre of Tokyo. It became the first line to open under the urban-railway convenience-enhancement framework established by a 2005 law, and was Sōtetsu's first wholly new line in about 43 years, since the opening of the Sōtetsu Izumino Line. Construction was split into two phases, each carried out by JRTT: the Nishiya–Hazawa Yokohama-Kokudai first-phase line, and the Hazawa Yokohama-Kokudai–Shin-Yokohama second-phase line.

The line's name was settled on 13 December 2018, when the Nishiya–Shin-Yokohama route was announced as the "Sōtetsu Shin-Yokohama Line." On 28 March 2019 the operators announced that the first section, from Nishiya to Hazawa Yokohama-Kokudai, would open on 30 November 2019. That section duly opened on schedule, immediately launching through-services that ran from the Sōtetsu Main Line (Nishiya–Ebina) over JR East's freight line and onto the Shōnan-Shinjuku Line, Saikyō Line and Kawagoe Line toward central Tokyo and Saitama. It was the first brand-new railway line to open in Japan in the Reiwa era, and platform doors entered use at the new Hazawa Yokohama-Kokudai Station (numbered SO51) on opening day.

Naming on the line is deliberately precise. In Sōtetsu's own usage and in the official Railway Directory (Tetsudō Yōran) the line is always written with the company prefix as the "Sōtetsu Shin-Yokohama Line," never simply "Shin-Yokohama Line," in order to avoid confusion with the separate Tōkyū Shin-Yokohama Line. JRTT, as owner, instead labels the two construction phases the "Sōtetsu–JR Link Line" (the Nishiya–Hazawa section) and the "Sōtetsu–Tōkyū Link Line" (the Hazawa–Shin-Yokohama section, built jointly with the Tōkyū Shin-Yokohama Line). Through-running stock was introduced ahead of each stage: the Sōtetsu 20000 series for Tōkyū services from February 2018, the 12000 series for JR services from April 2019, and the 21000 series for Tōkyū Meguro Line services from September 2021.

Work then turned to the second phase toward Shin-Yokohama. On 27 January 2022 the operators formally announced that the Hazawa Yokohama-Kokudai–Shin-Yokohama section would open in March 2023 and set out the planned through-service network, and a rail-fastening ceremony was held inside Shin-Yokohama Station on 22 July 2022. Station numbering was decided on 16 September 2022, giving Shin-Yokohama the number SO52, and the station's formal name was confirmed on 21 October 2022. Familiarisation test running over the new section began on 3 November 2022, and on 16 December 2022 the operators announced the opening date of 18 March 2023.

The Hazawa Yokohama-Kokudai–Shin-Yokohama section opened as announced on 18 March 2023, connecting at Shin-Yokohama to the Tōkyū Shin-Yokohama Line. From that date through-services ran from the Sōtetsu Main Line at Ebina and from the Sōtetsu Izumino Line at Shōnandai, via the Tōkyū Shin-Yokohama Line, onto the Tōkyū Meguro Line and the Toei Mita, Tokyo Metro Namboku and Saitama Rapid Railway lines, and onto the Tōkyū Tōyoko, Tokyo Metro Fukutoshin and Tōbu Tōjō lines. The Japanese Wikipedia article records that the opening formed a network of seven railway operators and fourteen lines totalling about 250 kilometres. With the Tōkyū Shin-Yokohama Line, it became the first case in the Kantō region of two different major private railways being directly linked without passing through a subway operator.

The Sōtetsu Shin-Yokohama Line is mostly in tunnel and has no level crossings at all. The new facilities at Shin-Yokohama Station are jointly operated by Sōtetsu and Tōkyū, and the article notes that this was the first time either company had a station with a direct connection to the Shinkansen — Shin-Yokohama being a stop on the Tōkaidō Shinkansen. A completion ceremony was held at Shin-Yokohama on 5 March 2023, and the use of platform doors at Shin-Yokohama on the 18 March opening completed platform-door installation at every station on the line. Today the line carries frequent through-services that knit eastern Kanagawa, Yokohama, central Tokyo and Saitama into a single one-seat network.

• 201811 February: the Sōtetsu 20000 series enters service, intended for through-running onto the Tōkyū Tōyoko Line.
• 201813 December: the Nishiya–Shin-Yokohama route is officially named the Sōtetsu Shin-Yokohama Line.
• 201928 March: the opening date for the Nishiya–Hazawa Yokohama-Kokudai section is announced as 30 November 2019.
• 201920 April: the Sōtetsu 12000 series enters service, intended for through-running onto the JR Saikyō Line.
• 201930 November: the Nishiya–Hazawa Yokohama-Kokudai section opens with through-service to JR East's Shōnan-Shinjuku, Saikyō and Kawagoe lines; it is the first new railway line to open in Japan in the Reiwa era. Hazawa Yokohama-Kokudai (SO51) opens with platform doors.
• 202123 May: platform doors enter use at Nishiya Station.
• 20216 September: the Sōtetsu 21000 series enters service, intended for through-running onto the Tōkyū Meguro Line.
• 202227 January: the operators formally announce that the Hazawa Yokohama-Kokudai–Shin-Yokohama section will open in March 2023, together with the planned through-service network.
• 202222 July: a rail-fastening ceremony is held inside Shin-Yokohama Station.
• 202216 September: station numbering is decided; Shin-Yokohama is given the number SO52.
• 202221 October: the formal station name for Shin-Yokohama is confirmed.
• 20223 November: familiarisation test running begins over the Hazawa Yokohama-Kokudai–Shin-Yokohama section.
• 202216 December: the opening date for the Hazawa Yokohama-Kokudai–Shin-Yokohama section is announced as 18 March 2023.
• 20235 March: a completion ceremony is held at Shin-Yokohama Station.
• 202318 March: the Hazawa Yokohama-Kokudai–Shin-Yokohama section opens, linking via the Tōkyū Shin-Yokohama Line to the Tōkyū Meguro/Tōyoko lines and the Toei Mita, Tokyo Metro Namboku/Fukutoshin, Saitama Rapid Railway and Tōbu Tōjō lines; platform doors at Shin-Yokohama complete the line. A network of seven operators and fourteen lines (~250 km) is formed.
• 202416 March: Tōkyū Meguro Line stock begins entering the Sōtetsu Atsugi Line via the Sōtetsu Main Line, on deadhead (out-of-service) trains only.