Tozai Line
The Tokyo Metro Tozai Line (東西線 Tōzai-sen) runs, as its name implies (literally "East-West Line") from east to west across the centre of Tokyo, unless you are going the other way. Though as it moves west from the terminus at Nishi-Funabashi across the vastness of Edokawa-ku it moves south, before starting to creep generally northwards as it intersects the Yamanote Line loop. In fact pedantic purists could well insist that the Toei Shinjuku Line is the truer "east-west line", as it moves in this direction with much fewer diversions to the north or south.
No self-respecting metro line would be complete without interconnections to other lines, and the Tozai Line is no slacker here. At the western end some trains continue from Nakano on the Chuo-Sobu Line as far as Mitaka, while on the eastern side interrunning is in operation with the obscure Toyo Line and at peak periods with the Sobu Line as far as Tsudanuma.
Fascinating Tozai Line Fact
The Tozai Line is the only metro line in Tokyo which has direct connections with all other lines, except the newly-opened Fukutoshin Line, which strikes me as a silly omission on the part of the planners, but there you go.