Keio Corporation

The line was extended toward both Shinjuku and the west, and the Shinjuku–Fuchū section was fully double-tracked in 1923. Fuchū–Higashi-Hachiōji was built separately by the Gyokunan Electric Railway, an affiliate established under the Local Railways Act in an unsuccessful bid for a government subsidy; it opened on 24 March 1925 to the 1,067 mm gauge. Keio absorbed Gyokunan on 1 December 1926, regauged the section to 1,372 mm by June 1927, and ran through trains over the whole Shinjuku–Higashi-Hachiōji line from 22 May 1928. The interwar company broadened its business: it sold electricity to surrounding towns and villages, opened the Goryō Line in 1931, entered the real-estate business in 1938, and renamed stations after sightseeing spots — Seiseki-Sakuragaoka, Takahata-Fudō, Mogusaen — names that survive today.

The Inokashira Line has a separate ancestry: it was opened by the Teito Electric Railway (帝都電鉄), a company in the Kinugawa Hydroelectric group, which completed Shibuya–Inokashira-kōen on 1 August 1933 and reached Kichijōji on 1 April 1934. Teito was merged into the Odawara Express Railway on 1 May 1940; in 1941 that company's parent, Kinugawa Hydroelectric, absorbed it and took the name Odakyu Electric Railway, which was in turn merged into Tokyo Kyuko Electric Railway — the wartime 'Dai-Tokyu' combine — on 1 May 1942. In 1942 Keio Electric Tramway had to hand its bus routes inside the city of Tokyo to the city and, under the Electricity Distribution Control Ordinance, its entire power-supply business to Kantō Haiden, a heavy blow to the company; on 31 May 1944 it was itself absorbed by Tokyu under the Land Transportation Business Coordination Act and dissolved. The war years brought air-raid damage, the relocation of the Shinjuku terminal to its present site in July 1945, and the Keio Line's conversion from tramway to railway status that October.

On 26 December 1947 a Tokyu shareholders' meeting voted to break up the combine by spinning off four companies, and on 1 June 1948 the Keio and Inokashira lines began operations under the new Keio Teito Electric Railway (京王帝都電鉄), with capital of 50 million yen and 1,944 employees. Adding the Inokashira Line, a plan promoted by Sadao Inoue, then head of Tokyu's Keio branch and later Keio Teito's president, was meant to give the weakened ex-tramway a viable base, though the new company's early results were the poorest among the major private railways. Growth came through diversification — Keio Foods (1959, now Keio Store), the Keio Department Store (1961), the Keio Plaza Hotel company (1969) — and steady building: the Keibajō Line opened in 1955; 1963 brought the underground Shinjuku terminal, the raising of the line voltage to 1,500 V and the first limited expresses; the Dōbutsuen Line opened in 1964 and the Takao Line in 1967; the Sagamihara Line was extended from 1971 as the access route to Tama New Town, reaching Hashimoto on 20 March 1990; and the Keio New Line of 1978 enabled through services with the Toei Shinjuku Line from 16 March 1980. Keio was also among the first railways to introduce priority seating, inaugurating its 'Silver Seats' (renamed priority seats in 1993) on Respect for the Aged Day, 15 September 1973.

On 1 July 1998, in the 50th anniversary year of its establishment, Keio Teito Electric Railway dropped the Teito name to become Keio Electric Railway (京王電鉄); the English name was changed to Keio Corporation on 29 June 2005. The modern company runs a network in two systems: the Keio Line system, whose tramway-era 1,372 mm gauge is now unique among the railway lines of the major private railways, and the 1,067 mm gauge Inokashira Line, which crosses it at Meidaimae. Recent decades brought the PASMO IC card (2007), ATC on every line (completed 2013), station numbering (2013), the undergrounding of the Chōfu area (2012), and — on 7 September 2012 — becoming the first major private railway whose revenue trains all used VVVF inverter control. The centenary of train and bus operations was marked on 15 April 2013, and the second-generation 5000 series of 2017 enabled the 'Keio Liner' reserved-seat services launched on 22 February 2018. In the year to March 2021, amid the COVID-19 pandemic, all five major Tokyo-area private railways including Keio posted net losses. The second-generation 2000 series entered service on 31 January 2026.

• 190512 December: the Nippon Electric Railway applied to the authorities for permission to build an electric railway — the earliest origin of today's Keio Electric Railway.
• 191012 April: the company was renamed Keio Electric Tramway; 21 September: it was formally incorporated with capital of 1.25 million yen.
• 191315 April: Keio Electric Tramway opened its first section, Sasazuka–Chōfu (12.2 km, 1,372 mm gauge, under the Tramways Act), starting a bus service over the unbuilt section the same day.
• 192524 March: the affiliated Gyokunan Electric Railway opened Fuchū–Higashi-Hachiōji to the 1,067 mm gauge under the Local Railways Act.
• 19261 December: Keio Electric Tramway absorbed the Gyokunan Electric Railway; the ex-Gyokunan section was regauged to 1,372 mm by 1 June 1927, and through running between Shinjuku and Higashi-Hachiōji began on 22 May 1928.
• 19331 August: the Teito Electric Railway opened Shibuya–Inokashira-kōen, the origin of the Inokashira Line; the full line to Kichijōji was completed on 1 April 1934.
• 194431 May: under the Land Transportation Business Coordination Act, Keio Electric Tramway was merged into Tokyo Kyuko Electric Railway, becoming part of the wartime 'Dai-Tokyu' combine.
• 19481 June: following the December 1947 Tokyu shareholders' decision to break up the combine, the Keio and Inokashira lines began operations under the new Keio Teito Electric Railway (capital 50 million yen, 1,944 employees).
• 19631 April: the underground Shinjuku terminal entered service; 4 August: the overhead-line voltage was raised from 600 V to 1,500 V and the first-generation 5000 series entered service; 1 October: limited-express (tokkyū) operation began between Shinjuku and Higashi-Hachiōji.
• 198016 March: reciprocal through services between the Keio Line and the Toei Shinjuku Line began; the Keio New Line, which made this possible, had opened on 31 October 1978.
• 199020 March: the Sagamihara Line was completed with the opening of Minami-Ōsawa–Hashimoto; the line had been extended in stages since 1971 as the access route to Tama New Town.
• 19981 July: marking the 50th anniversary of its establishment, Keio Teito Electric Railway was renamed Keio Electric Railway (京王電鉄); the English company name became Keio Corporation on 29 June 2005.
• 20127 September: Keio became the first major private railway whose revenue trains were all VVVF-inverter-controlled; on 19 August the line through the Chōfu area (Shibasaki–Nishi-Chōfu and Chōfu–Keio-Tamagawa) had been moved underground.
• 201822 February: the 'Keio Liner' reserved-seat services began, operated with the second-generation 5000 series introduced on 29 September 2017.