Toei Ōedo Line

The project's origins reach back to the Urban Transportation Council's Report No. 6 of 8 June 1962, which proposed a "Line 9" running from the Roka-kōen area via Hōnan-chō, Shinjuku, Kasuga-chō, Umayabashi, Fukagawa and Tsukishima toward Azabu. Keio opposed the western alignment because it ran through an important Keio Bus operating area, and on 27 March 1964 the council adopted Keio's counter-proposal, rerouting the Roka-kōen–Shinjuku section along a quadruple-tracked Keio Line via Sakurajōsui, Meidaimae and Hatagaya. In the council's Report No. 10 of 10 April 1968 the former Line 9 was renumbered Line 12, and Report No. 15 of March 1972 reshaped it into a loop returning to Shinjuku, with an extension to the Takamatsu (Hikarigaoka) district of Nerima.

A railway-construction licence for the whole line, between present-day Hikarigaoka and Tochōmae and covering both the radial and circular portions, was obtained in August 1974, with the aim of opening the full line in 1985 using 20-metre cars in ten-car formations like the Shinjuku Line. The sharp change in conditions after the 1973 oil crisis and the Transportation Bureau's worsening finances froze the construction plan from the middle of 1976; an application filed in December 1975 to build a large-capacity line between Nerima and Hikarigaoka was never reviewed and lapsed automatically. The decision to redevelop the former Grant Heights site at Hikarigaoka as a large housing complex revived the need for transit, and in December 1982 the Tokyo Metropolitan Government resolved to build Line 12.

Construction between Hikarigaoka and Nerima began on 1 June 1986, and the line's colour was fixed as magenta that same month. To hold construction costs down, the line adopted iron-wheel linear-motor "mini-subway" technology — the second such line in Japan after the Osaka municipal subway's Nagahori Tsurumi-ryokuchi Line — following test running on a converted 12-000 series prototype and a formal decision to use the system in December 1988. The first segment, from Hikarigaoka to Nerima, opened on 10 December 1991 under the working name "Toei Line 12", and from its first winter the line has run all-night services on New Year's Eve every year.

The line then grew in stages. The radial arm was extended from Nerima to Shinjuku on 19 December 1997, at which point the whole fleet was lengthened from six-car to eight-car formations, and from Shinjuku to Kokuritsu-Kyōgijō on 20 April 2000. It was at this April 2000 extension that the line received its present name, "Ōedo Line" — meaning "Great Edo", after Tokyo's former name. As with earlier lines, the public had been polled to choose a name, but Governor Shintarō Ishihara rejected the winning entry, "Tokyo Loop Line" (東京環状線), arguing that the line would not at first form a complete loop and that the name would be confused with the Yamanote Line and the Osaka Loop Line.

The loop was closed on 12 December 2000, when the Kokuritsu-Kyōgijō–Roppongi–Tochōmae section opened and the whole line entered service. The opening date — 12/12 of Heisei 12 in the Japanese calendar — was chosen because the route was "Line 12", and a commemorative "Ōedo Line Queen" special departed Tochōmae at 12:12. Shiodome Station was held back at the full opening because the surrounding area was still being redeveloped and its access road was unfinished; trains passed through the site, used as a signal post, until Shiodome opened as a station on 2 November 2002. Because the line is completely underground, English Wikipedia describes it as the second-longest railway tunnel in Japan after the Seikan Tunnel.

The Ōedo Line is the deepest-running of Tokyo's subways, and many of its stations lie well below ground; the line runs as low as about 48 m below the surface at points and crosses beneath the Sumida River three times. Measured from the surface to platform level, Roppongi is the deepest station on the whole Toei network, its platform 1 lying 42.3 m down. Its linear-motor technology is incompatible with conventional rotary-motor stock, and its small tunnels and loading gauge bar other trains, so the Ōedo Line is the only Toei subway line that runs no through services with other operators — the sole exception being a track connection to the Asakusa Line usable only by Class E5000 locomotives. Carrying the highest daily ridership in the Toei network, it remains a heavily used circular-and-radial spine through the heart of Tokyo.

• 19628 June: the Urban Transportation Council's Report No. 6 proposes a 'Line 9' (the future Line 12) from the Roka-kōen area via Shinjuku and the eastern wards toward Azabu.
• 196810 April: in Report No. 10 the former Line 9 is renumbered Line 12.
• 1972March: Report No. 15 reshapes the plan into a loop returning to Shinjuku and adds an extension toward the Takamatsu (Hikarigaoka) district of Nerima.
• 1974August: a railway-construction licence is obtained for the whole line, between present-day Hikarigaoka and Tochōmae, with an aim of opening in 1985.
• 1976Mid-year: amid the aftermath of the 1973 oil crisis and the Transportation Bureau's worsening finances, the Line 12 construction plan is temporarily frozen.
• 1982December: after the decision to redevelop the former Grant Heights site at Hikarigaoka, the Tokyo Metropolitan Government resolves to build Line 12.
• 19861 June: construction begins between Hikarigaoka and Nerima; the line's colour is fixed as magenta the same month.
• 198821 December: the Tokyo Metropolitan Government decides to adopt the iron-wheel linear-motor system for the line, after test running on a converted 12-000 series prototype.
• 199110 December: the first segment, Hikarigaoka–Nerima, opens under the working name 'Toei Line 12'.
• 199719 December: the line is extended from Nerima to Shinjuku; the whole fleet is lengthened from six-car to eight-car formations.
• 200020 April: the line is extended from Shinjuku to Kokuritsu-Kyōgijō and is renamed the 'Ōedo Line'; Governor Shintarō Ishihara had rejected the public-poll choice 'Tokyo Loop Line'.
• 200012 December: the Kokuritsu-Kyōgijō–Roppongi–Tochōmae section opens and the whole line enters service; the date 12/12 of Heisei 12 was chosen to match 'Line 12'.
• 20022 November: Shiodome Station, held back at the full opening because its surroundings were still under redevelopment, opens to connect with the Yurikamome.
• 2025Plans are reported to extend the line westward from Hikarigaoka to a new Ōizumigakuenchō terminus with three new stations, expected to open around 2040.