Toei Mita Line

The route grew out of Tokyo's postwar subway planning. It was first conceived in 1957 as a northern branch of Line 5, then made an independent line; on 8 June 1962 the Council for Urban Transport recommended it as "Line 6," and the Tokyo Metropolitan Government took on its construction, with the city-planning decision following in August 1962. Plans to through-run with private railways were drawn up — including a 1964 scheme to connect with the Tōbu Tōjō Line — but in 1965 Tokyu and Tobu decided against the through-service arrangement, and the Toei Line 6 was built as a self-contained route. Construction began in December 1965.

The line opened in stages. The first segment, between Sugamo and Shimura, ran 10.4 km and opened on 27 December 1968, worked at first by four-car trains of the new 6000 series. Shimura Station was renamed Takashimadaira in August 1969, and in July 1970 the line received its blue identifying colour, the 6000 series having its body stripe changed from red to blue. Six-car operation began in May 1972, and the line was then extended south in steps: 7.3 km from Sugamo to Hibiya on 30 June 1972, and a further 3.3 km from Hibiya to Mita on 27 November 1973. At the northern end, a 1.5 km extension from Takashimadaira to Nishi-Takashimadaira opened on 6 May 1976. The line was renamed the Toei Mita Line on 1 July 1978.

For the rest of the route the southern terminus stayed at Mita for over two decades while the extension to Meguro was planned. In 1985 the Council on Transport Policy confirmed the Meguro extension in its Report No. 7, and Toei reached a memorandum with the (then) Teito Rapid Transit Authority over the parallel Line 7 in 1988; the project licences were granted in 1989 and construction of the Mita–Meguro section began in 1992. Rolling stock was renewed in parallel: the 6300 series entered service on 23 June 1993, and the original 6000 series ended regular operation on 28 November 1999, after which the line ran on 6300-series trains alone.

The extension and the move to through-running were prepared together at the turn of the century. Through late 1999 and 2000 the line's systems were modernised — the signalling was changed from T-type ATS to a new CS-ATC, train radio was switched from inductive to space-wave radio, and automatic train operation (ATO) was introduced across the line in April 2000. The line's formal name was simplified from "Toei Mita Line" to "Mita Line" on 20 April 2000. On 22 September 2000 a timetable revision brought in driver-only (one-man) operation and the first runs of Tokyu trains.

The final 4.0 km segment, from Mita to Meguro, opened on 26 September 2000, completing the line, and on the same day reciprocal through service with the Tokyu Meguro Line began. From Meguro, Mita Line trains run onto the Tokyu Meguro Line, and Tokyu and other operators' trains run onto the Mita Line, knitting the once self-contained subway into the wider network of southwest Tokyo and Kanagawa. Through service was later extended as the Tokyu Meguro Line itself was extended to Hiyoshi in 2008.

In the line's later years the emphasis has been on platform safety and longer trains. Platform-edge barriers were trialled at Takashimadaira in 1999 and installation began across the line that year, with all stations equipped by 10 August 2000; the doors were subsequently renewed, with the replacement of platform doors at all 24 stations completed on 11 May 2022 to suit eight-car trains. The new 6500 series entered service on 14 May 2022, introducing eight-car operation. On 18 March 2023 the through-running network was widened again when service over the Tokyu Shin-Yokohama Line and Sotetsu Shin-Yokohama Line to the Sotetsu lines began.

• 19628 June: the Council for Urban Transport recommends the route as "Line 6"; in August the city-planning decision is made and the Tokyo Metropolitan Government takes on its construction.
• 1965Tokyu and Tobu decide against the planned through service, so Toei Line 6 is built as a self-contained route; construction begins in December.
• 196827 December: the first segment, Sugamo–Shimura (10.4 km), opens; four-car 6000 series trains begin service.
• 19691 August: Shimura Station is renamed Takashimadaira.
• 1970July: the line receives its blue identifying colour; the 6000 series body stripe is changed from red to blue.
• 197214 May: six-car operation begins; 30 June: the line is extended south 7.3 km from Sugamo to Hibiya.
• 197327 November: the line is extended a further 3.3 km south from Hibiya to Mita.
• 19766 May: a 1.5 km extension from Takashimadaira to Nishi-Takashimadaira opens at the northern end.
• 19781 July: the line is renamed the Toei Mita Line.
• 198511 July: the Council on Transport Policy confirms the extension to Meguro in its Report No. 7.
• 199323 June: the 6300 series enters revenue service.
• 199928 November: the 6000 series ends regular operation; from the following day the line runs on 6300-series trains alone.
• 200020 April: the formal name is simplified to 'Mita Line'; April: ATO automatic operation begins line-wide; 22 September: a timetable revision brings in driver-only operation and the first Tokyu-train runs.
• 200026 September: the final 4.0 km segment from Mita to Meguro opens, completing the line, and reciprocal through service with the Tokyu Meguro Line begins.
• 200822 June: through service is extended as the Tokyu Meguro Line is extended to Hiyoshi.
• 202211 May: replacement of platform doors at all 24 stations is completed for eight-car trains; 14 May: the 6500 series enters service, introducing eight-car operation.
• 202318 March: through service to the Sotetsu lines via the Tokyu Shin-Yokohama Line and Sotetsu Shin-Yokohama Line begins.