Kaikyō Line

Before the line existed, rail traffic between Aomori and Hakodate crossed the Tsugaru Strait on the Seikan ferry (Seikan renraku-sen) operated by Japanese National Railways. The strait crossing was repeatedly disrupted — by drifting mines that entered the strait in the 1950s, thought to be connected with the Korean War, and above all by the 1954 Tōya Maru disaster, in which a ferry that sailed under a mistaken weather judgement during an approaching typhoon was wrecked. A wartime-era idea of joining Honshu and Hokkaido by tunnel gained sudden momentum, and the Seikan Tunnel was eventually built as a more dependable alternative to shipping, at the cost of a long construction period and an enormous outlay.

The project had a long official gestation. An informal Tsugaru Strait Connecting Tunnel Committee was set up within the Ministry of Transport in February 1946, and years of land and seabed geological surveys followed; the route was added to the list of planned lines in 1953. It was entered as a survey line in 1961 and designated a construction line in 1970, with the formal start-of-work ceremonies held on both the Honshu and Hokkaido sides in November 1971. Originally designed to conventional-line standards, the line was redesigned to Shinkansen standards to suit the nationwide Seibi-Shinkansen plan; after that plan was frozen it opened provisionally as a conventional line, and — gauge and catenary voltage aside — it became a prototype for the later "Super Express" (suupaa tokkyū) concept.

Following the privatisation of JNR, the Kaikyō Line opened on 13 March 1988, the same day the Seikan Tunnel itself was opened, as the JR line connecting Honshu and Hokkaido; regular sailings of the Seikan ferry ended that same day. The line never carried trains under its own name in everyday timetables: together with the adjoining sections of JR East's Tsugaru Line and JR Hokkaido's Kōsei Line (later the Dōnan Isaribi Railway Line) and Hakodate Main Line, over which all its trains ran through, it was marketed under the nickname "Tsugaru-Kaikyō Line." Two unusual stations, Tappi-Kaitei and Yoshioka-Kaitei, were built inside the tunnel, doubling as tunnel emergency facilities and serving only sightseeing visitors rather than ordinary passengers.

The line was engineered for fast running and was steadily sped up. Maximum speed was 120 km/h at opening; on 16 March 1991 it was raised to 140 km/h between Naka-Oguni and Kikonai, shortening Aomori–Hakodate limited-express journeys by about seven minutes. A new passenger station, Chinai, opened on 1 July 1990 by upgrading the Shin-Yunosato Signal Station. On the freight side, the conversion of Kaikyō Line freight to full containerisation was completed with the timetable revision of 1 March 1994, after which carload working all but disappeared; container trains have predominated ever since, and the carriage of some dangerous goods through the tunnel is restricted for safety.

Conventional passenger service then declined in stages. With the December 2002 timetable revision the Rapid "Kaikyō" was withdrawn and ordinary trains vanished from the Tsugaru-Kaikyō Line, leaving only express and limited-express services. The undersea stations were wound down: Yoshioka-Kaitei lost its scheduled stops on 18 March 2006 and was passed by all trains from 28 August that year, and Tappi-Kaitei was passed by all trains from 11 November 2013. On 15 March 2014 Tappi-Kaitei, Yoshioka-Kaitei and Chinai were all abolished — the two undersea stations becoming the Tappi and Yoshioka "fixed points" (teiten) kept as emergency refuges, and Chinai becoming Yunosato-Chinai Signal Station.

The decisive change came with the Hokkaidō Shinkansen. In preparation for its opening, standard-gauge rails were laid alongside the existing narrow-gauge track to create dual-gauge sections; the catenary voltage on the Shin-Naka-Oguni–Kikonai section was raised from 20 kV to 25 kV AC on 22 March 2016, and the line's analogue ATC was replaced by digital DS-ATC. The last conventional passenger trains — the limited expresses "Hakuchō" and "Super Hakuchō" and the "Hamanasu" express — ran on 21 March 2016. On 26 March 2016 the Hokkaidō Shinkansen opened between Shin-Aomori and Shin-Hakodate-Hokuto, the Shin-Naka-Oguni–Kikonai section became shared standard-and-narrow-gauge track, Tsugaru-Imabetsu Station was abolished and reopened as the Shinkansen's Okutsugaru-Imabetsu Station, and the line's scheduled conventional passenger trains ceased.

Today the Kaikyō Line is in everyday terms a freight-only route, the artery carrying goods between Honshu and Hokkaido by a path far less weather-dependent than air or sea. Because the upgraded electrification and signalling barred the older electric and electric-locomotive-hauled trains, JR Freight introduced dual-voltage Class EH800 locomotives to keep running through, and the surviving Kaikyō Line passenger platform at Kikonai was removed in 2017. The only passenger trains that now use the line are charter specials such as the luxury cruise train Train Suite Shikishima, which began running in May 2017. Owing to the shared track, the same rails carry both a local-line fare for the conventional Kaikyō Line and a trunk-line fare for the Shinkansen. JR Hokkaido has filed to abolish the 2.3-kilometre Naka-Oguni–Shin-Naka-Oguni overlap with the Tsugaru Line, planned for 1 April 2027, though that section will survive physically as part of JR East's Tsugaru Line.

• 1946February: an informal Tsugaru Strait Connecting Tunnel Committee is established within the Ministry of Transport, beginning years of geological surveys.
• 195426 September: the Tōya Maru ferry disaster during a typhoon adds urgency to building a Honshu–Hokkaido tunnel.
• 1971November: start-of-work ceremonies for the Seikan Tunnel are held on both the Honshu and Hokkaido sides (designated a construction line in 1970).
• 198813 March: the Kaikyō Line opens between Naka-Oguni and Kikonai (87.8 km) with the Seikan Tunnel; the Seikan ferry's regular service ends. Maximum speed is 120 km/h at opening.
• 19901 July: the Shin-Yunosato Signal Station is upgraded to a passenger station and opens as Chinai Station.
• 199116 March: maximum speed between Naka-Oguni and Kikonai is raised from 120 km/h to 140 km/h, cutting Aomori–Hakodate limited-express times by about seven minutes.
• 19941 March: with the timetable revision, full containerisation of Kaikyō Line freight is completed; carload working all but disappears.
• 20021 December: the Rapid 'Kaikyō' is withdrawn and ordinary trains disappear from the Tsugaru-Kaikyō Line, leaving only express and limited-express services.
• 200618 March: Yoshioka-Kaitei loses its scheduled stops; from 28 August it is passed by all trains and becomes a seasonal station.
• 201311 November: with its tour course ending, Tappi-Kaitei Station is passed by all trains.
• 201415 March: Tappi-Kaitei, Yoshioka-Kaitei and Chinai are abolished; the two undersea stations become the Tappi and Yoshioka 'fixed points' (emergency refuges) and Chinai becomes Yunosato-Chinai Signal Station.
• 201621 March: the last conventional passenger trains — limited expresses 'Hakuchō'/'Super Hakuchō' and the 'Hamanasu' express — run on the line.
• 201622 March: the catenary voltage on the Shin-Naka-Oguni–Kikonai section is raised from 20 kV to 25 kV AC for Shinkansen compatibility; analogue ATC is replaced by digital DS-ATC.
• 201626 March: the Hokkaidō Shinkansen opens (Shin-Aomori–Shin-Hakodate-Hokuto); the Shin-Naka-Oguni–Kikonai section becomes shared dual-gauge track, Tsugaru-Imabetsu is abolished and reopens as the Shinkansen's Okutsugaru-Imabetsu, and scheduled conventional passenger trains cease.
• 2017May: passenger running resumes only as charter specials, with the luxury cruise train Train Suite Shikishima; the remaining Kaikyō Line passenger platform at Kikonai is removed the same year.
• 20271 April (planned): JR Hokkaido is to abolish the 2.3 km Naka-Oguni–Shin-Naka-Oguni overlap with the Tsugaru Line; the section survives physically as part of JR East's Tsugaru Line.