Seibu Kokubunji Line

The line opened on 21 December 1894, when the Kawagoe Railway laid an 8.0-kilometre line from Kokubunji to a temporary Kumegawa station — the site of present-day Higashimurayama — as the first section of its planned route to Kawagoe. At first this Kokubunji–Kawagoe route was the company's main line. The Kawagoe Railway was a subsidiary of the Kōbu Railway, the predecessor of today's Chūō Main Line, and through trains were operated between Iidamachi and Kawagoe by way of Kokubunji. The line was built without electrification and was at first worked by steam.

On 21 March 1895 the railway was extended from the temporary Kumegawa station to Kawagoe — the present Hon-Kawagoe — adding 21.7 kilometres and completing the through route from Kokubunji. In 1911 the line was designated a light railway. The Kawagoe Railway was absorbed into Musashi Suiden in 1920, and in 1922 the railway business was split off to become a line of the (first) Seibu Railway, beginning the corporate lineage that the modern operator traces back to this route.

The line's role was transformed in 1927. On 16 April that year the Murayama Line — the present-day Shinjuku Line — opened between a temporary Takadanobaba station and Higashimurayama, and through services began running between Takadanobaba and Kawagoe over the new alignment. The original Kokubunji–Higashimurayama stretch was thereby separated from the main flow of traffic and left as a branch, taking essentially the form it has today. In 1945 the (first) Seibu Railway was merged into the Musashino Railway, and the line passed to the company that became Seibu Agricultural Railway, renamed Seibu Railway in 1946.

Electrification came in stages. The Higashimurayama–Kawagoe section had been electrified at 1,500 V DC on 16 April 1927, but the Kokubunji–Higashimurayama branch was not electrified until 5 November 1948. Intermediate stations were added to the Kokubunji end over time — Takanodai opened in 1948 and Koigakubo in 1955 — while the line gained the name it carries today on 25 March 1952, when the Higashimurayama–Hon-Kawagoe section was absorbed into the Shinjuku Line and the remaining Kokubunji–Higashimurayama portion was renamed the Kokubunji Line. The single-track line was partly double-tracked on 12 November 1968, when the stretch between Hanesawa Signal Box and Koigakubo was doubled.

For much of its later history the Kokubunji Line ran through trains onto the Shinjuku Line. A timetable revision on 12 March 2003 introduced daytime through services as far as Shin-Tokorozawa, and from 14 June 2008 these were extended to Hon-Kawagoe, restoring a daytime through link over the old Kawagoe Railway route. Construction to raise Higashimurayama Station onto an elevated structure began in January 2015, and on 16 March 2019 the through services to the Shinjuku Line were suspended as the grade-separation works progressed; the elevation of the area around Higashimurayama is planned to be switched over at the end of the 2026 fiscal year.

Today the Kokubunji Line is operated as a self-contained shuttle: all trains are local services that call at every station and shuttle between Kokubunji and Higashimurayama, taking about twelve minutes for the 7.8-kilometre run. Because station platforms hold only six cars, the line is worked by six-car trains, drawn mainly from Seibu's 2000 series in their distinctive yellow livery. On 31 May 2025 the first Seibu 8000 series train — a set converted from a former Odakyu 8000 series and acquired as a second-hand 'Sustaina' vehicle — entered service on the line as part of the gradual replacement of the 2000 series, opening a new chapter for the oldest line on the Seibu network.

• 189111 April: a provisional railway construction licence is granted to the Kawagoe Railway.
• 189221 June: the Kawagoe Railway is granted its railway construction licence.
• 189421 December: the Kawagoe Railway opens its first section, Kokubunji to the temporary Kumegawa station (now Higashimurayama), 8.0 km, non-electrified.
• 189521 March: the line is extended from the temporary Kumegawa station to Kawagoe (now Hon-Kawagoe), 21.7 km, completing the through route from Kokubunji.
• 191116 February: the line is designated a light railway.
• 1920The Kawagoe Railway is merged into Musashi Suiden.
• 1922The railway business is split off to become a line of the (first) Seibu Railway.
• 192716 April: the Higashimurayama–Kawagoe section is electrified at 1,500 V DC; the Murayama Line (now the Shinjuku Line) opens from a temporary Takadanobaba to Higashimurayama, and through running begins on the new route — leaving Kokubunji–Higashimurayama as a branch.
• 1945The (first) Seibu Railway is merged into the Musashino Railway, becoming Seibu Agricultural Railway (renamed Seibu Railway in 1946).
• 19485 November: the Kokubunji–Higashimurayama section is electrified at 1,500 V DC; intermediate Takanodai station opens.
• 195225 March: the Higashimurayama–Hon-Kawagoe section is absorbed into the Shinjuku Line and the remaining Kokubunji–Higashimurayama portion is renamed the Kokubunji Line.
• 1955Koigakubo station opens as an intermediate station.
• 196812 November: the Hanesawa Signal Box–Koigakubo section is double-tracked.
• 200312 March: a timetable revision introduces daytime through services to the Shinjuku Line as far as Shin-Tokorozawa.
• 200814 June: the daytime through services are extended to Hon-Kawagoe.
• 201916 March: through services to the Shinjuku Line are suspended as the Higashimurayama Station grade-separation works progress.
• 202531 May: the first Seibu 8000 series train (converted from a former Odakyu 8000 series) enters service, beginning the replacement of the 2000 series.