Marunouchi Line Hōnanchō Branch

The branch was conceived as part of a postwar plan to extend the Marunouchi Line west from Shinjuku. A 1957 Construction Ministry notice, based on an Urban Transport Council report, laid out Line 4 as a main line running west along Ōme-kaidō toward Ogikubo to relieve crowding on the Japanese National Railways Chūō Line, together with a 2.7-kilometre branch from the Nakano-sakaue area to Hōnanchō. The branch was intended both to serve the area around the planned Nakano depot and to fill a gap in railway coverage in that part of Nakano. The new western sections, including the branch, were opened under the separate name Ogikubo Line.

Operating Agency of Underground Railways (the Teito Rapid Transit Authority, or TRTA) decided to build the Shinjuku–Ogikubo main line and the Nakano-sakaue–Hōnanchō branch on 18 May 1957, obtained the local-railway construction licence on 1 March 1958, and broke ground on the western works on 14 March 1959. On 21 October 1958 the company decided to call the line west of Shinjuku the Ogikubo Line, a name it would carry for more than a decade.

The branch opened in two stages. On 8 February 1961 the first section, from Nakano-sakaue to Nakano-fujimichō (1.9 km), opened together with the Shinjuku–Shin-Nakano stretch of the main Ogikubo Line; about every other Marunouchi Line train from the Ikebukuro–Shinjuku section ran through onto the new line, while trains turning back within the branch itself ran as short two-car sets. On 23 March 1962 the remaining section from Nakano-fujimichō to Hōnanchō (1.3 km) opened, completing the Ogikubo Line. Because passenger demand on the branch was light, it was for many years worked by short trains transferred from the Ginza Line — TRTA 100-series cars from 1962 to 1968, and then TRTA 2000-series cars.

From the outset the Marunouchi Line and the Ogikubo Line were operated as a single integrated system, and on 1 April 1972 the Ogikubo Line name was abolished and the line was merged into the Marunouchi Line, forming the network that exists today. The branch saw a series of incremental upgrades over the following decades: in 1981 its trains were lengthened from two-car to three-car formations to add capacity, and in 1996 the dedicated 02-series 80-subseries cars entered service on it, replacing the last of the old red 500-series rolling stock on the branch that July.

In the 2000s the branch received modern safety and operating equipment ahead of the main line. Movable platform-edge gates (platform doors) were installed along the Nakano-sakaue–Hōnanchō section on 8 May 2004, and on 31 July 2004 the branch became the first part of the Marunouchi Line to switch to one-person (driver-only) operation, applying at first to the three-car shuttle trains that ran only within the branch. In November 2016 the branch's station-numbering prefix was changed from a lowercase "m" to "Mb," so that automated announcements would distinguish its stations from similarly named ones on the main line.

The branch's role changed markedly at the end of the 2010s. After the platforms at Hōnanchō were lengthened to take six-car trains, six-car through services between Ikebukuro and Hōnanchō began on 5 July 2019, giving the branch direct trains to the city centre for the first time. From 13 March 2021 some of the branch's own turn-back services also began running as six-car trains in parts of the morning and evening, and on 27 August 2022 the last three-car 02-series trains were withdrawn from the branch and all services were standardised on six-car formations. Today the branch is worked by Tokyo Metro 2000-series trains, the same type used across the rest of the Marunouchi Line.

• 195718 May: TRTA (the Teito Rapid Transit Authority) decides to build the Shinjuku–Ogikubo main line and the Nakano-sakaue–Hōnanchō branch.
• 19581 March: the local-railway construction licence for the Shinjuku–Ogikubo and Nakano-sakaue–Hōnanchō sections is obtained. On 21 October the line west of Shinjuku is named the Ogikubo Line.
• 195914 March: construction begins on the Shinjuku–Ogikubo and Nakano-sakaue–Hōnanchō works.
• 19618 February: the first branch section, Nakano-sakaue–Nakano-fujimichō (1.9 km), opens together with the Shinjuku–Shin-Nakano stretch of the Ogikubo Line; branch turn-back trains run as two-car sets.
• 196223 March: the Nakano-fujimichō–Hōnanchō section (1.3 km) opens, completing the Ogikubo Line and the Hōnanchō branch.
• 1968May–July: the branch's ten TRTA 100-series cars (transferred earlier from the Ginza Line) are replaced by ten TRTA 2000-series cars.
• 19721 April: the Ogikubo Line name is abolished and the line is merged into the Marunouchi Line, forming the present network.
• 198116 November: the branch's trains are lengthened from five two-car sets to six three-car sets to increase capacity.
• 19963 July: dedicated 02-series 80-subseries trains enter service on the branch; on 18 July the last 500-series cars are withdrawn from the branch, retiring the old red trains from the whole line.
• 20048 May: movable platform-edge gates are installed on the Nakano-sakaue–Hōnanchō section. 31 July: the branch becomes the first part of the line to adopt one-person (driver-only) operation, for its three-car in-branch shuttles.
• 2016November: the branch's station-numbering prefix is changed from lowercase "m" to "Mb" so automated announcements distinguish its stations from similarly named main-line stations.
• 20195 July: after the platforms at Hōnanchō are lengthened for six-car trains, six-car through services between Ikebukuro and Hōnanchō begin. (EN Wikipedia, citing a 2013 forecast, gives 'fiscal 2017'; the JA-sourced actual start date is used here.)
• 202113 March: some of the branch's own turn-back services begin running as six-car trains in parts of the morning and evening.
• 202227 August: the last three-car 02-series trains are withdrawn from the branch and all services are standardised on six-car formations.