In 1952, a dual-track extension from Takadanobaba to Seibu-Shinjuku Station was completed. At this time the line was renamed the Shinjuku Line, integrating the Murayama Line and the northern section of the Kawagoe Line. The new Seibu-Shinjuku terminal was built as a temporary station, as Seibu planned to extend the line to the second floor of what is now known as Lumine Est on the east side of Shinjuku Station. This plan was later scrapped due to insufficient space to handle trains longer than six cars. Seibu-Shinjuku Station was expanded to include a high-rise hotel in 1977.