The is a rapid transit line in Tokyo and Chiba Prefecture, Japan, owned and operated by Tokyo Metro. Its name translates to "East-West Line". The line runs between Nakano in Nakano-ku, Tokyo and Nishi-Funabashi in Funabashi, Chiba Prefecture. The Tōzai Line was referred to as Line 5 during the planning stages; the seldom-used official name is . The line carries an average of 1,642,378 passengers daily (2017), making it the busiest line on the Tokyo Metro network. On maps, diagrams and signboards, the Tōzai Line is shown using the color "sky blue" ( ; #009bbf) and its stations are given numbers using the letter "T". Overview The line runs through central Tokyo from east to west via Takadanobaba, Waseda, Ōtemachi, Nihombashi, Kiba and Urayasu. It was opened as a bypass route for the Chuo Rapid Line and the Sobu Line, which had been incredibly congested at the time. It is the only Tokyo Metro line to extend into Chiba Prefecture (although the Shinjuku Line operated by Toei also extends into Chiba Prefecture.) The Tōzai Line features through services at both ends of the line. Trains run onto the JR East Chūō-Sōbu Line for at the western (Nakano) end, and onto either the Chūō-Sōbu Line for or the Tōyō Rapid Railway Line for at the eastern (Nishi-Funabashi) end. According to the Tokyo Metropolitan Bureau of Transportation in 2018, the Tokyo Metro Tōzai Line continues to be most crowded subway line in Tokyo, and the most crowded train line in all of Japan, with its peak running at 199% capacity between and stations. Women-only cars were introduced on the line for use during morning rush hour on November 20th, 2006. During the COVID-19 pandemic, peak ridership dropped from a rate of 199% in 2019 to 123% in 2020. History The Tōzai Line was planned by a review committee of the then Ministry of Transportation in 1962 and numbered Line 5. Its name literally means "East-West Line", and it was primarily planned to relieve traffic on the busy Sōbu Main Line as well as provide a straight crosstown connection through north-central Tokyo. Although this corridor is also served by the Tokyo Metropolitan Bureau of Transportation (Toei) Shinjuku Line and JR Keiyō Line, the Tōzai Line continues to operate beyond capacity due to its accessibility to other lines, as well as to growing condominium developments in eastern Tokyo. The to section opened in 1964,