Seibu Sayama Line

The line was not originally built by Seibu but by the Musashino Railway, the present company's predecessor, which opened it in 1929. At first it was known as the Musashino Railway Yamaguchi Line and ran somewhat farther than the line does today, terminating at a station near the reservoirs northwest of Tokorozawa. Like several lightly used branch lines, it was suspended during the Second World War as a "non-urgent" line, and it did not return to service until the early 1950s, reopening in a shortened and renamed form.

The Musashino Railway opened the Yamaguchi Line between Nishi-Tokorozawa and Murayama-kōen, a distance of 4.8 km, on 1 May 1929, electrified at 1,200 V DC. Over the following years its intermediate and terminal stations were repeatedly renamed as the surrounding reservoirs and parkland developed: in 1933 Kami-Yamaguchi became Yamaguchi-Chosuichi and Murayama-kōen became Murayama-Chosuichi-sai, and in 1941 both reverted, to Kami-Yamaguchi and Murayama respectively. On 28 February 1944 the entire Nishi-Tokorozawa–Murayama section was suspended as a wartime non-urgent line.

The line was revived after the war. On 7 October 1951 the Nishi-Tokorozawa–Sayama-ko section reopened, the route was renamed the Sayama Line, and Sayama-ko Station was relocated; services resumed using diesel railcars rather than electric trains. Electric operation returned on 21 March 1952 when the line was re-electrified, this time at 1,500 V DC. The two stations that had stood on the original alignment did not all come back at once: on 10 October 1954 the still-suspended Shimo-Yamaguchi and Kami-Yamaguchi stations were formally abolished, and Shimo-Yamaguchi was not reopened until 4 June 1976, while Kami-Yamaguchi was never restored.

The line's modern character was shaped by baseball. In 1978 the former Seibuen Stadium near the terminus was rebuilt, in near-new form, as Seibu Lions Stadium (today the Belluna Dome) to serve as the home ground of the Seibu Lions, and the Sayama Line was substantially upgraded as the access route to the ballpark. The terminus itself was relocated on 30 November 1978, and on 25 March 1979 Sayama-ko Station was renamed Seibukyūjō-mae ("in front of the Seibu Stadium"). On game and event days the short line still carries heavy crowds, with limited-express trains — normally absent from the line — and special through services from the Ikebukuro and Shinjuku lines laid on for spectators.

Day-to-day, the Sayama Line is operated mostly as a self-contained shuttle between Nishi-Tokorozawa and Seibukyūjō-mae, a journey of about six minutes, with passengers changing at Nishi-Tokorozawa for Ikebukuro Line trains. A former rapid-express (kaisoku-kyūkō) service on the line was discontinued in the timetable revision of 26 March 1998. From 6 March 2010 regular through trains to the Ikebukuro Line were introduced and the daytime frequency was raised from one train every 20 minutes to one every 15 minutes; in the timetable revision of 12 March 2022 the daytime interval was returned to 20 minutes.

Station numbering was introduced across all Seibu Railway lines during fiscal 2012, the Sayama Line's stations taking the "SI" prefix shared with the Seibu Ikebukuro group of lines: Nishi-Tokorozawa (SI18), Shimo-Yamaguchi (SI40) and Seibukyūjō-mae (SI41). More recently the line has been chosen for fleet renewal: from 27 June 2026 Seibu plans to introduce its 7000 series — former Tokyu 9000 series cars acquired as "Sustaina" rolling stock — on local services, and to begin camera-assisted one-man operation on those trains the same day.

• 19291 May: the Musashino Railway opens its Yamaguchi Line between Nishi-Tokorozawa and Murayama-kōen (4.8 km), electrified at 1,200 V DC.
• 19331 March: Kami-Yamaguchi Station is renamed Yamaguchi-Chosuichi and Murayama-kōen Station is renamed Murayama-Chosuichi-sai.
• 19411 April: Yamaguchi-Chosuichi Station reverts to Kami-Yamaguchi and Murayama-Chosuichi-sai Station reverts to Murayama.
• 194428 February: the Nishi-Tokorozawa–Murayama section is suspended as a wartime non-urgent line.
• 19517 October: the Nishi-Tokorozawa–Sayama-ko section reopens and the line is renamed the Sayama Line; Sayama-ko Station is relocated and services restart with diesel railcars.
• 195221 March: the line is re-electrified at 1,500 V DC, restoring electric operation.
• 195410 October: the still-suspended Shimo-Yamaguchi and Kami-Yamaguchi stations are abolished.
• 19764 June: Shimo-Yamaguchi Station reopens.
• 1978Seibuen Stadium near the terminus is rebuilt as Seibu Lions Stadium (now the Belluna Dome), home of the Seibu Lions, and the line is heavily upgraded as the access route; the terminus is relocated on 30 November.
• 197925 March: Sayama-ko Station is renamed Seibukyūjō-mae.
• 199826 March: the line's former rapid-express (kaisoku-kyūkō) service is discontinued in the timetable revision.
• 20106 March: regular through trains to the Ikebukuro Line are introduced and the daytime interval is shortened from 20 to 15 minutes.
• 2012Station numbering is introduced on all Seibu lines during fiscal 2012; Sayama Line stations take the 'SI' prefix (Nishi-Tokorozawa SI18, Shimo-Yamaguchi SI40, Seibukyūjō-mae SI41).
• 202212 March: the daytime interval is returned from 15 to 20 minutes in the timetable revision.
• 202627 June (planned): Seibu introduces its 7000 series — ex-Tokyu 9000 series 'Sustaina' cars — on local services and begins camera-assisted one-man operation on those trains.