is a private railway operator in Tokyo, Japan and the central firm of the that is involved in transport, retail, real estate and other industries. The Keio railway network connects western suburbs of Tokyo (Chà Âfu, Fuchà «, Hachià Âji, Hino, Inagi, Tama) and Sagamihara in Kanagawa with central Tokyo at Shinjuku Station.
The name is derived from taking one character each from the places through which the railway runs: and .
The Keio network is based around the central Keià  Line, , 32 stations.
The Keio Inokashira Line does not share track with the Keio Main Line. It intersects with the Keio Line at Meidaimae Station.
The company's earliest predecessor was the founded in 1905. In 1906 the company was reorganized as the , and in 1910 was renamed yet again to . It began operating its first stretch of interurban between Sasazuka and Chà Âfu in 1913. By 1923, Keià  had completed its main railway line (now the Keià  Line) between Shinjuku and Hachià Âji. Track along the Fuchà « – Hachià Âji section was originally laid in 1,067 mm gauge by the ; it was later changed to match the rest of the line's 1,372 mm gauge.
The Inokashira Line began operating in 1933 as a completely separate company, . This company had also planned to link with Suzaki (now Kà Âtà  ward), though this never materialized. In 1940, Teito merged with the Odakyu Electric Railway, and in 1942 the combined companies were merged by government order into (now Tokyu Corporation).
In 1947, the shareholders of Tokyu voted to spin off the Keio and Inokashira lines into a new company, . The Teito name was dropped in 1998 in favor of , though "KTR" placards and insignia can still be seen occasionally. The company's English name was changed to Keio Corporation on June 29, 2005.
Keià  was among the first railway companies to introduce priority seats on its trains. Priority seats are those reserved for the physically handicapped, elderly, pregnant women, and people with infants. These special seats, which were initially called "Silver seats" but renamed in 1993, were inaugurated on Respect for the Aged Day on September 15, 1973.
All Keio trains have longitudinal (commuter-style) seating.
The first of a fleet of five new ten-car 5000 series EMUs was introduced on 29 September 2017, ahead of the start of new evening reserved-seat commuter services from Shinjuku in spring 2018.