Sapporo Municipal Subway Tōzai Line

Planning for an east–west subway dates to the 1960s, when the city drew up a high-speed transit construction programme; the line was envisaged as a counterpart to the north–south Namboku Line, the first stage of which had opened in time for the 1972 Sapporo Winter Olympics. On 4 October 1971 the Sapporo city assembly approved construction of the first section, between Kotoni and Shiroishi. The municipality obtained a local-railway construction licence for that section on 12 May 1973, held a groundbreaking ceremony on 29 March 1973, and began work on 18 May 1973. With the central segment under way, the assembly went on to approve the eastward extension from Shiroishi to Shin-Sapporo on 24 February 1975.

The Tōzai Line opened on 10 June 1976 between Kotoni and Shiroishi, a distance of 9.9 km. From the outset it was operated as a driver-attended but automatically driven railway, using Automatic Train Operation, with a fleet of four-car 6000 series trains (twenty sets, eighty cars). Like the Namboku Line before it, the new route used rubber-tyred rolling stock rather than steel wheels, a defining characteristic of the Sapporo subway. The line gave the rapidly growing western and central districts of Sapporo a fast cross-town link through the city centre at Ōdōri, where it crossed the existing Namboku Line.

On 21 March 1982 the line was extended eastward by 7.4 km from Shiroishi to Shin-Sapporo, giving the Tōzai Line its eastern terminus and a connection to the JR Chitose Line. To handle the longer route every train was lengthened to six cars, and additional 6000 series stock was built, bringing the fleet to twenty-four sets of 144 cars. A westward extension beyond Kotoni was also planned, but on 1 September 1987 the city decided to freeze it; the parallel JR Hakodate Main Line had gained new stations and more frequent service nearby, and concerns about ridership and investment returns led to the plan being shelved for the time being. Earlier the same year, on 22 March 1987, the Ōdōri–Nishi-Jūitchōme section had been closed temporarily, with substitute bus services, so that a connecting track to the new Tōhō Line could be built.

After about 1990 the automatic (ATO) operation of the 6000 series trains was discontinued and the line reverted to manual driving. The shelved western extension was revived during the 1990s: the project to reach Teine-higashi (a provisional name) was re-authorised on 13 February 1994, a railway business licence for the Kotoni–Teine-higashi section followed on 31 March 1994, and construction began on 16 March 1995. New 8000 series trains entered service on 18 August 1998, and on 25 February 1999 the final 2.8 km from Kotoni to Miyanosawa opened, giving the line its present western terminus. To serve the longer line the 6000 series sets were lengthened to seven cars by inserting 8300 series intermediate cars.

From 2002 the ageing 6000 series cars began to be replaced by 8000 series stock, and 8300 series cars that had been added to the 6000 series sets were transferred to the new trains. Modernisation continued through the 2000s: English-language and advertising announcements began on the 8000 series on 27 September 2005, and from 2006 cars modified for one-man (driver-only) operation entered service. The last 6000 series trains were withdrawn on 30 August 2008, completing the changeover to the 8000 series, and on 1 September 2008 automatic operation under ATO resumed — roughly eighteen years after it had been suspended.

The line's final transition to modern operation came in 2009. The SAPICA contactless smart card was introduced on 30 January 2009; platform edge doors, first trialled at Nangō-Nana-Chōme Station in 2008, were installed at every station by 3 March 2009; and on 1 April 2009 the Tōzai Line switched to wanman (driver-only) operation. A "Women and Children Comfort Car" was designated on each train from 13 July 2009. In 2023 the line reported around 219,400 boardings per day, roughly 99 percent of its 2019 total, making it a busy artery of the Sapporo subway alongside the Namboku and Tōhō lines.

• 19714 October: the Sapporo city assembly approves construction of the first Kotoni–Shiroishi section.
• 197312 May: a local-railway construction licence is obtained for Kotoni–Shiroishi (groundbreaking 29 March; work begins 18 May).
• 197524 February: the city assembly approves the eastward extension from Shiroishi to Shin-Sapporo.
• 197610 June: the line opens between Kotoni and Shiroishi (9.9 km); four-car 6000 series trains debut and run under ATO automatic operation.
• 198221 March: the Shiroishi–Shin-Sapporo section (7.4 km) opens; trains are lengthened to six cars and the fleet grows to 24 sets / 144 cars.
• 198722 March: the Ōdōri–Nishi-Jūitchōme section is temporarily closed (with bus substitution) for Tōhō Line connecting-track work; on 1 September the western extension beyond Kotoni is frozen.
• 199413 February: the shelved western extension toward Teine-higashi is re-authorised; a railway business licence for Kotoni–Teine-higashi follows on 31 March.
• 199818 August: the new 8000 series trains enter service.
• 199925 February: the Kotoni–Miyanosawa section (2.8 km) opens, completing the line; 6000 series sets are lengthened to seven cars with inserted 8300 series cars.
• 2002The ageing 6000 series cars begin to be replaced by 8000 series stock.
• 200527 September: English-language and advertising announcements begin on the 8000 series.
• 200830 August: the last 6000 series trains are withdrawn, completing the changeover to the 8000 series; on 1 September ATO automatic operation resumes after about 18 years.
• 200930 January: the SAPICA smart card is introduced; 3 March: platform edge doors are completed at all stations; 1 April: the line switches to wanman (driver-only) operation.