Toei Shinjuku Line

The line's defining technical feature is its track gauge. Unlike every other Tokyo subway line, which uses either 1,435 mm standard gauge or 1,067 mm narrow gauge, the Shinjuku Line was built to 1,372 mm — the so-called "Tokyo gauge" — making it the only subway in Japan laid to that gauge. The choice was forced by the plan to through-run onto the Keiō network. Tokyo and the then Ministry of Transport initially wanted 1,435 mm so the line could share equipment with the Toei Asakusa Line, and asked Keiō Teito Electric Railway (today Keiō Corporation) to regauge its Keiō Line; Keiō declined, citing the scale of the conversion work and concern over capacity during it, so the subway was instead built to match Keiō at 1,372 mm. Keiō's own 1,372 mm gauge traces back to the Tokyo Horse Railway, which adopted it in 1882 and passed it to the Tokyo city tram network that the early Keiō company had once hoped to join.

The project began as a planning-document route. In April 1968 the Urban Transportation Council recommended Line 10 as a route from the Roka-kōen area via Shinjuku to the Sumiyoshichō area, and in December 1968 the Shinjuku–Sumiyoshichō section was fixed in the city plan as Line 10. In May 1969 the planned route was extended to incorporate the Keiō Line between Chōfu and Shinjuku and pushed east toward Higashi-Ōjima, the site of the line's depot. Construction began near Morishita and Sumiyoshi on 1 May 1971. Shortly before opening, on 1 July 1978, the line was renamed from the provisional "Line 10" to the Toei Shinjuku Line, at the same time the former Lines 1 and 6 became the Toei Asakusa and Toei Mita lines.

The Shinjuku Line opened in stages over more than a decade. The first section, Iwamotochō–Higashi-Ōjima (6.8 km), opened on 21 December 1978. On 16 March 1980 the Shinjuku–Iwamotochō section (7.3 km) opened and through running with Keiō Teito Electric Railway began, via the Keiō New Line that had entered service in October 1978; at first, because the line east of Iwamotochō could only handle six-car trains, eight- and ten-car Keiō trains terminated there. The line then crept eastward: Higashi-Ōjima–Funabori (1.7 km) opened on 23 December 1983, and Funabori–Shinozaki (4.9 km) on 14 September 1986.

The final section, Shinozaki–Moto-Yawata (2.8 km), opened on 19 March 1989, completing the full Shinjuku–Moto-Yawata route, though Moto-Yawata at first opened only as a temporary station. The permanent Moto-Yawata Station opened on 1 September 1991, and on the same day Keiō trains' through-running range was extended all the way to Moto-Yawata. Through the late 1980s the platforms were progressively lengthened so that Keiō trains could run further east — reaching Ōjima in December 1987 — and ten-car Keiō operation, begun on a few trains in September 1981, gradually became standard.

Service patterns developed alongside the network. Express running on the line began on 24 December 1997, initially on weekday daytimes only, and through services to Takaosanguchi on the Keiō Takao Line started at the same time. On 20 April 2000 the line's formal name was simplified from "Toei Shinjuku Line" to simply "Shinjuku Line", and on 12 December 2000 the Toei Ōedo Line opened and met the Shinjuku Line at Morishita, which then gained express stops. Later timetable revisions repeatedly reshaped the Keiō through-services, including the express pattern between Sasazuka and Moto-Yawata and seasonal through-running to the Keiō Dōbutsuen Line.

Today the Shinjuku Line is one of Toei's busiest and most profitable routes; in fiscal 2023 it carried the second-highest daily ridership in the Toei network after the Ōedo Line. It is worked by Toei 10-300 series trains together with Keiō 9000 and 5000 series stock used on the through-services, and since 11 August 2022 all trains have run as ten-car formations. Its unusual gauge keeps it permanently tied to the Keiō network it was built to serve, and its eastern reach into Chiba makes it one of the few Tokyo subway lines that crosses a prefectural boundary under its own tracks.

• 196810 April: the Urban Transportation Council (Report No. 10) recommends Line 10, from the Roka-kōen area via Shinjuku to the Sumiyoshichō area.
• 196828 December: the Shinjuku–Sumiyoshichō section (12.5 km) is fixed in the city plan as Line 10.
• 196920 May: the planned route is extended to incorporate the Keiō Line between Chōfu and Shinjuku and to reach Higashi-Ōjima, the site of the depot.
• 19711 May: construction begins near Morishita and Sumiyoshi.
• 19781 July: the provisional 'Line 10' is renamed the Toei Shinjuku Line, alongside the renaming of the former Lines 1 and 6 to the Toei Asakusa and Toei Mita lines.
• 197821 December: the first section, Iwamotochō–Higashi-Ōjima (6.8 km), opens.
• 198016 March: the Shinjuku–Iwamotochō section (7.3 km) opens and through running with Keiō Teito Electric Railway begins via the Keiō New Line; eight- and ten-car Keiō trains initially terminate at Iwamotochō.
• 19811 September: ten-car Keiō trains begin operating on some services.
• 198323 December: the Higashi-Ōjima–Funabori section (1.7 km) opens.
• 198614 September: the Funabori–Shinozaki section (4.9 km) opens.
• 198720 December: platforms are lengthened line-wide, extending the Keiō trains' through-running range from Iwamotochō to Ōjima.
• 198919 March: the final section, Shinozaki–Moto-Yawata (2.8 km), opens, completing the line; Moto-Yawata opens as a temporary station.
• 19911 September: the permanent Moto-Yawata Station opens, and Keiō trains' through-running range is extended to Moto-Yawata.
• 199724 December: express service begins on the line (weekday daytimes only at first); through running to Takaosanguchi on the Keiō Takao Line also starts.
• 200020 April: the line's formal name is simplified from 'Toei Shinjuku Line' to 'Shinjuku Line'. 12 December: the Toei Ōedo Line opens and connects at Morishita, which gains express stops.
• 202211 August: the last eight-car Toei trains are withdrawn and all trains run as ten-car formations.