Nanakuma Line

The line links central Fukuoka with the southwestern part of the city, an area that previously had no rail-based transport. It was first envisaged decades before it opened: a 1971 report by the Urban Transport Council recommended a new high-speed railway, and through the 1980s a succession of regional transport and city-planning studies — including a 1989 Kyushu regional transport council report — endorsed a radial line connecting the southwest and centre. A full-scale planning survey for 'Subway Line 3' was carried out between 1992 and 1994, and in December 1994 the Hashimoto–Tenjin-Minami project was adopted for national subsidy.

A railway business licence for the Hashimoto–Tenjin-Minami section was applied for in March 1995 and granted in June 1995; construction began in December 1996, with a safety-prayer and groundbreaking ceremony held in January 1997. The route through the central area, between Ropponmatsu and Tenjin-Minami, was driven beneath Jōnan-dōri and Watanabe-dōri, roads once used by the Nishitetsu Fukuoka city tram. The line's popular name was decided in 2003 through a public competition: 'Jōnan Line' came first and 'Fukudai Line' second, but the third-placed 'Nanakuma Line' was chosen because Nanakuma is a historic place-name dating back to the Kamakura period and lies near the centre of the route.

The Hashimoto–Tenjin-Minami section opened on 3 February 2005, running from Hashimoto in the southwest to Tenjin-Minami in the city centre. From the outset every station was fitted with platform screen doors, and the stations and equipment were designed around a thoroughgoing universal-design scheme that won a Japan Sign Design Association grand prize in 2005 and a Japan Society of Civil Engineers design award in 2010; the rolling stock received the Railfan Club's Laurel Prize in 2006. Unusually for an earlier Japanese linear-motor mini-subway, the line's Hashimoto depot was built at grade rather than underground, giving the system its first above-ground stretch, albeit on a non-revenue section.

Almost from the start there was strong demand to extend the line from Tenjin-Minami to Hakata, the city's main terminal, by way of the busy Canal City Hakata commercial complex. The city decided on a Canal City routing after a 2010 cost-benefit study, obtained a railway business permit for the 1.6 km Tenjin-Minami–Hakata extension in June 2012, and held a groundbreaking ceremony in February 2014; the extension was originally expected to open around fiscal 2020.

The extension works were dogged by ground collapses. Sinkholes had already occurred during construction in 2000 and again during the extension works in 2014, but by far the most serious came on 8 November 2016, when a large road collapse roughly 27 metres wide, 30 metres long and 15 metres deep opened near the Hakataeki-mae 2-chōme intersection in front of JR Hakata Station. Underground electricity, gas, water and sewer mains were severed, nearby businesses and lifelines were disrupted, surrounding roads were closed all day and an evacuation advisory was issued; the city acknowledged the same day that its subway works were the cause and apologised. A government expert panel later attributed the collapse chiefly to a thinner-than-expected and weathered rock layer above the tunnel combined with water-bearing sandy ground. Because of the accident the planned opening was pushed back from fiscal 2020 to fiscal 2022, and ultimately to March 2023.

The Tenjin-Minami–Hakata extension opened on 27 March 2023, adding Kushida Shrine Station and Hakata Station and bringing the line to its full 18 stations and 13.6 km. It was the first subway line to open anywhere in Japan in the Reiwa era. The new section dives deep beneath the Naka and Hakata rivers and existing buildings, so the platforms at Kushida Shrine and Hakata lie more than 25 metres underground. With the extension, the off-gate interchange that had let passengers transfer between the Nanakuma Line at Tenjin-Minami and the Airport Line at Tenjin was in principle abolished, and Hakata became the line's main connection to the wider rail network. In November 2022 the city announced that it was studying a further extension eastward from Hakata to the international terminal at Fukuoka Airport.

• 197111 March: an Urban Transport Council report recommends the construction of a new high-speed railway, an early step toward what becomes Line 3.
• 198916 October: a Kyushu regional transport council report endorses studying a radial line linking the city's southwest-centre with the city centre.
• 1994December: the Hashimoto–Tenjin-Minami project is adopted for national subsidy, following a full-scale planning survey carried out in 1992–1994.
• 1995A railway business licence for the Hashimoto–Tenjin-Minami section is applied for on 28 March and granted on 7 June.
• 199611 December: construction begins on the Hashimoto–Tenjin-Minami section.
• 199722 January: a safety-prayer and groundbreaking ceremony is held for the Hashimoto–Tenjin-Minami works.
• 200020 June: a road collapse occurs at a construction site near the Jōtōbashi-nishi intersection on Jōnan-dōri (about 5 m × 10 m × 8 m), the first of three sinkholes associated with the line's works.
• 200320 June: the line's popular name is decided as 'Nanakuma Line,' chosen from a public competition over the first-placed 'Jōnan Line' and second-placed 'Fukudai Line.'
• 20053 February: the Hashimoto–Tenjin-Minami section opens, Japan's fourth iron-wheel linear-motor mini-subway; all stations have platform screen doors from the outset.
• 2006The line's 3000 series rolling stock receives the Laurel Prize from the Japan Railfan Club.
• 20112 March: station numbering is displayed at all stations on the Nanakuma, Airport and Hakozaki lines.
• 201211 June: a railway business permit is obtained for the Tenjin-Minami–Hakata extension.
• 201412 February: a groundbreaking ceremony is held for the Tenjin-Minami–Hakata extension; on 27 October a road collapse (about 3 m × 3 m × 3 m) occurs at the extension site near the Hakata police-station intersection.
• 20168 November: a large road collapse (about 27 m × 30 m × 15 m) opens near the Hakataeki-mae 2-chōme intersection in front of JR Hakata Station during the extension works; mains are severed and an evacuation advisory is issued, and the city acknowledges its subway works as the cause.
• 2018Following the 2016 sinkhole, the planned opening of the extension is pushed back from fiscal 2020 to fiscal 2022 (later announced as March 2023).
• 202327 March: the Tenjin-Minami–Hakata extension opens, adding Kushida Shrine and Hakata stations to give 18 stations and 13.6 km — the first subway line to open in Japan in the Reiwa era; the off-gate Tenjin/Tenjin-Minami interchange is in principle abolished.