Tosa Kuroshio Railway Asa Line

The line is the surviving fragment of a far larger plan. Under Table No. 107 of the revised Railway Construction Act of 1922, the state designated a predetermined line running "from Gomen in Kōchi Prefecture via Aki and Hiwasa in Tokushima Prefecture to the vicinity of Furushō" — a route that, with the Pacific coast around Cape Muroto, formed part of a long-cherished "Shikoku Loop Railway" concept linking Kōchi and Tokushima. The Tokushima end of this corridor between Muki, Hiwasa and Furushō had already been built between 1936 and 1942 and today forms part of JR Shikoku's Mugi Line. The remaining stretch, planned to connect Muki with Gomen at roughly 113 kilometres, became the Japanese National Railways construction line known as the Asa Line.

The Asa Line advanced slowly through the bureaucratic stages of railway planning: a predetermined line in 1922, a survey line in 1957, and a construction line in 1959, before being placed under the Japan Railway Construction Public Corporation as a works line in 1964. Construction began at the ends that connected to existing track. On the Tokushima side, work started from Muki, and on 1 October 1973 the Muki–Kaifu section (11.6 km) opened as part of the JNR Mugi Line. On the Kōchi side — the segment known as the Asa-sai (West Asa) Line — groundbreaking on the Aki–Tano stretch took place in 1965 under the loop-railway concept, and viaduct work between Gomen and Aki followed from April 1974.

That earlier stretch between Gomenmachi and Aki had been served since 1930 by the Tosa Electric Railway's Aki Line. By the early 1970s that interurban was running deep deficits, and in March 1974 it was abolished — its closure framed, with reference to the precedent of the Kōjaku Railway, as a way to clear deficits through the sale of its right-of-way and to promote construction of the national Asa Line that would replace it.

The project then collapsed. In December 1981, under the Special Measures Act for the Promotion of JNR Management Reconstruction, construction of the Asa-sai Line was frozen, as was work on the Tokushima side. By that point about 18.9 kilometres — roughly 44 percent of the route — had been completed, and the half-built viaducts and embankments stood abandoned for years across the Kōchi countryside, earning the route nicknames such as Tosa's "Great Wall" and the "Tower of Babel." The unbuilt central section between Kōnoura, Cape Muroto and Nahari was never started, and it remains an uncompleted line to this day, with the gap covered by local buses.

Rescue came through the third-sector model. In February 1986 a meeting of the heads of the municipalities along the Asa and Sukumo lines agreed to establish a single third-sector company, and on 8 May 1986 the Tosa Kuroshio Railway was founded, led by Kōchi Prefecture, to take over construction of both the abandoned Asa and Sukumo lines. The new company obtained its Class-1 railway licence for the Gomen–Nahari section in January 1988, and construction resumed in March of that year. Because the firm first concentrated on building the Sukumo Line and on sustaining the Nakamura Line, work on the Asa-sai Line was deferred. On the Tokushima side, the separate Asa Kaigan Railway opened the Kaifu–Kōnoura section (8.5 km) as its Asatō Line on 26 March 1992.

The Gomen–Nahari section finally opened on 1 July 2002 — about thirty-seven years after the first groundbreaking, and twenty-eight years after the closure of the Aki Line it succeeded. It was the last of the public corporation's regional development ("AB") lines to open apart from those whose construction was cancelled outright, and, as of the 2020s, the last newly built non-electrified railway line to open in Japan. A fleet of stainless-steel 9640 series diesel railcars — "9640" can be read as "Kuroshio," the company's name — was built for the opening, including two open-deck observation cars whose rounded, whale-motif ends were designed for sightseeing along the coast.

The Gomen-Nahari Line is perhaps best known for its station characters. Every station was given an image character designed by the Kōchi-born manga artist Yanase Takashi, the creator of Anpanman; the characters appear as objets at the stations, as markings on the trains, and on merchandise, and in 2004 a local citizens' group made costumes for them and toured events as the "Gotogoto Kigurumi-tai." The cast ranges from "Gomen Ekio-kun," a station attendant at Gomen, to "Naha Riko-chan" at Nahari, and after Yanase's death the character "Aki Nurse-chan" for the new Aki General Hospital-mae Station, opened in March 2021, was designed by Yanase's studio.

• 192211 April: the Muki–Gomen route is incorporated as a predetermined line under Table No. 107 of the revised Railway Construction Act, within the 'Shikoku Loop Railway' concept.
• 19573 April: the Asa Line is designated a survey line.
• 19599 November: the Muki–Gomen route is promoted to a construction line.
• 196422 April: the line is placed under the Japan Railway Construction Public Corporation as a works line.
• 1965March: under the loop-railway concept, construction of the Aki–Tano section of the Kōchi-side (Asa-sai) line begins as a new JNR line.
• 19731 October: the Muki–Kaifu section (11.6 km) opens as part of the JNR Mugi Line.
• 1974March: the Tosa Electric Railway Aki Line (Gomenmachi–Aki, opened 1930) is abolished; April: viaduct work on the Gomen–Aki stretch begins.
• 1981December: construction of the Asa-sai (West Asa) Line is frozen under the Special Measures Act for the Promotion of JNR Management Reconstruction, with about 18.9 km (≈44%) complete.
• 19868 May: the Tosa Kuroshio Railway is established, led by Kōchi Prefecture, to resume construction of the abandoned Asa and Sukumo lines.
• 198828 January: Tosa Kuroshio Railway is granted its Class-1 railway licence for the Gomen–Nahari section; 23 March: construction resumes.
• 199226 March: on the Tokushima side, the Asa Kaigan Railway opens the Kaifu–Kōnoura section (8.5 km) as the Asatō Line.
• 20016 December: track-laying on the Gomen–Nahari section is completed.
• 20021 July: the Gomen–Nahari section (42.7 km) opens as the Tosa Kuroshio Railway Asa Line (Gomen-Nahari Line) — the last regional 'AB' development line to open apart from cancelled ones.
• 20061 March: station numbering is introduced at all stations on the line.
• 202113 March: Aki General Hospital-mae Station (GN27-1) opens between Kyūjōmae and Aki; its character 'Aki Nurse-chan' was designed by Yanase's studio after Yanase Takashi's death.