The is a subway line in Tokyo, Japan, operated by Tokyo Metro. The line runs in a U-shape between Ogikubo Station in Suginami and Ikebukuro Station in Toshima, with a branch line between Nakano-Sakaue Station and Hōnanchō Station. The official name is . The line was named after the Marunouchi business district in Chiyoda, Tokyo, under which it passes. On maps, diagrams and signboards, the line is shown using the color red (), and its stations are given numbers using the letters "M" for the main line and "Mb" for the branch line. Overview The Marunouchi Line is the second line to be built in the city, and the first one constructed after the Second World War. The route is U-shaped, running from Ogikubo Station in the west of the city via the commercial and administrative district of Shinjuku through to the Marunouchi commercial center around Tokyo Station, before turning back and heading to Ikebukuro. Along with the Ginza Line, it is self-enclosed and does not have any through services with other railway lines. The Marunouchi Line is served by Tokyo Metro 02 series rolling stock in six-car trains on the main line, and three-car trains on the Hōnanchō branch. The main line is the most frequent subway line in Tokyo, with trains running at intervals of 1 minute 50 seconds during peak hours. In spite of such high-frequency service, according to a 2008 survey by the Ministry of Land, Infrastructure, Transport and Tourism the Marunouchi Line is one of the most crowded railway lines in Tokyo, but in the 2018 release, the Marunouchi line is one of the more crowded Metro lines, running at 169% capacity between Shin-ōtsuka and Myōgadani stations. Its age and relatively short train length has made it one of the most crowded lines in Tokyo, although the 2000 opening of the Toei Ōedo Line has relieved the problem somewhat. In response to crowding, Tokyo Metro upgraded all stations with chest-high platform doors on March 28, 2009, a date on which it also began driver-only operation. The Hōnanchō branch switched to driver-only operation in July 2004. Due to the age of the Marunouchi Line and the relative shallowness at which it runs, at several points in central Tokyo trains run at or above ground level. These include Yotsuya Station, the Kanda River near Ochanomizu Station (see image), and between Kōrakuen and Myōgadani stations. On maps, diagrams