[3] The deepest section is at the immediately adjacent Higashi-Shinjuku Station, where the line goes down to 35 meters (115 ft), partly due to an underground space reservation for a possible future extension of the Jōetsu Shinkansen to Shinjuku.

During these hours only women, children of elementary school age or younger and physically disabled passengers (and their carers) may board the first carriage.

The line was initially planned in 1972 as a run from Shiki to Shinjuku, with the possibility of further extension to Shibuya, Shinagawa and Haneda Airport.

[11] A 3.2 km (2.0 mi) segment from Kotake-Mukaihara to Ikebukuro, running parallel to the Yurakucho Line on separate tracks began operation in 1994.

The newest segment connecting the districts of Shinjuku and Shibuya via Zōshigaya, Shinjuku-sanchōme, and Meiji-Jingūmae (‘Harajuku’) opened for service on 14 June 2008, officially completing the Fukutoshin Line.

[12] Service to the Senkawa and Kanamechō stations, which had been bypassed by the Yūrakuchō New Line, also started on the same day.