Seibu railway is a major private railway company operating in the north-west of Tokyo, with a network essentially composed of two mainlines, the Shinjuku Line and Ikebukuro Line, named after their Tokyo terminuses, and several branchlines, including some minor lines (like the Tamagawa Line) not connected to the rest of the network.
Seibu railway is the core company of the Seibu group, wich started as a railway-only company, but soon diversified it’s businesses (like many other private railways, such as Tokyu), and now also owns the Prince Hotel company and the Seibu Lions professional baseball team.
Ikebukuro Line Network
Ikebukuro Line, Chichibu Line, Sayama Line, Toshima Line
Shinjuku Line Network
Shinjuku Line, Hajima Line, Seibuen Line