Nakamura Line

The Nakamura Line was conceived as the railway authorised in the Railway Construction Act's appended schedule as the line "from near Kubokawa in Kōchi Prefecture to Nakamura." In effect it was built as an extension of the Japanese National Railways (JNR) Dosan Main Line — today's JR Shikoku Dosan Line — and was operated as one with it, but it was never absorbed into the Dosan Line and instead took its own independent line name. That formal separateness later became the reason it was singled out as a deficit line and, eventually, as a specified local line targeted for conversion. The line was built by the Japan Railway Construction Public Corporation, the body that constructed new national railways in the post-war decades.

The first section, from Kubokawa to Tosa-Saga (20.7 km), opened on 18 December 1963 as a diesel-operated line. As far as the junction at Kawaoku Signal Box — where the later Yodo Line would branch off — the stretch was treated as a continuation of the Dosan Main Line, and the section between Kubokawa and Wakai carried distance posts measured 200 km from Tadotsu, the Dosan Line's origin. The remaining section, from Tosa-Saga to Nakamura (22.7 km), opened on 1 October 1970, completing the through route to Nakamura. North of Wakai the line climbs through a spiral (loop) almost entirely hidden inside the 2,031-metre First Kawaoku Tunnel.

Almost from the start the line's finances were precarious. In September 1968 it was named one of the "83 deficit lines" identified for possible discontinuation, though that plan was cancelled in 1972. Limited express service nonetheless arrived: on 15 March 1972 the "Nanpū" limited express began running through from Takamatsu via Kubokawa to Nakamura. Centralised traffic control was introduced on 1 March 1974, and on 1 February 1984 freight operations on the line ceased. East-Ōgata Station was renamed Ariigawa on 15 November 1982.

Under the law to promote the rebuilding of JNR's finances, the Nakamura Line was designated in 1986 as a third-batch specified local transport line — a category of money-losing rural line marked for closure or conversion. It was selected on 7 April 1986, the transport minister approved its designation on 27 May, and a council agreed to convert it to a third-sector railway. The newly formed Tosa Kuroshio Railway held its first extraordinary shareholders' meeting on 22 November 1986 and resolved to take the line on; it was granted a first-class railway business licence on 18 December 1987.

With the privatisation of JNR on 1 April 1987 the line passed first to the Shikoku Railway Company (JR Shikoku), and then on 1 April 1988 its operation was transferred to Tosa Kuroshio Railway. At the handover the operating distance was revised, shortening the line by 0.4 km across its length to 43.0 km, and a new station, Kotsuka, was opened. The through limited expresses, which had run as the "Ashizuri" express in JNR days, were recast: the "Ashizuri" became a rapid service (and in part a local) before being restored to limited-express status on 21 November 1990. Two further intermediate stations were later added, Saga-Kōen on 1 October 1993 and Uminoōmukae on 22 April 2003.

The line gained its present operating character with the opening of the Tosa Kuroshio Railway's Sukumo Line on 1 October 1997, which extends the Nakamura Line southwest beyond Nakamura toward Sukumo. The Sukumo Line was built as a continuation of the Nakamura Line, and with its opening the maximum speed of limited expresses within the Nakamura Line was raised to 110 km/h; most of the through expresses began continuing to Sukumo. On 26 September 2015 the Nakamura and Sukumo lines were jointly given the public nickname "Shimanto Kuroshio Line." The line has also suffered weather damage: a large landslide between Kaina and Iyoki on 8 December 2003 cut services there until 10 January 2004, and a derailment caused by torrential rain on 2 June 2023 led to a suspension, with full service restored on 10 June 2023.

Today the Nakamura Line continues to host the JR Shikoku limited expresses that run through from Takamatsu and Kōchi over the Dosan Line — the "Shimanto" and "Ashizuri" — just as it did before conversion. Under the timetable in force from 16 March 2024 the line carries eight round-trip limited expresses, one "Shimanto" and seven "Ashizuri," of which one and a half round trips continue onto the Sukumo Line; to operate these through services Tosa Kuroshio Railway owns express diesel cars of the same JR Shikoku 2700 series type. The "Nanpū" limited express from Okayama also once ran through to the line, but that through-running ended with the timetable revision of 14 March 2020. Local trains run as one-man services between Kubokawa and Nakamura and on toward Sukumo.

• 196318 December: the first section, Kubokawa–Tosa-Saga (20.7 km), opens as a diesel-operated JNR line, in effect an extension of the Dosan Main Line.
• 1968September: the line is selected as one of the '83 deficit lines'; the discontinuation plan is cancelled in 1972.
• 19701 October: Tosa-Saga–Nakamura (22.7 km) opens, completing the through route to Nakamura.
• 197215 March: the 'Nanpū' limited express begins running through from Takamatsu via Kubokawa to Nakamura.
• 19741 March: centralised traffic control (CTC) is introduced on the line.
• 198215 November: East-Ōgata Station is renamed Ariigawa Station.
• 19841 February: freight operations on the line cease.
• 1986The line is designated a third-batch specified local transport line (selected 7 April; transport minister's approval 27 May); conversion to a third-sector railway is agreed.
• 198622 November: the newly formed Tosa Kuroshio Railway holds its first extraordinary shareholders' meeting and resolves to take on operation of the line.
• 19871 April: with the privatisation of JNR, the line is succeeded by the Shikoku Railway Company (JR Shikoku); Tosa Kuroshio Railway is granted a first-class railway business licence on 18 December.
• 19881 April: operation of the line is transferred to Tosa Kuroshio Railway; the operating distance is revised, shortening the line by 0.4 km to 43.0 km, and the new Kotsuka Station opens.
• 199021 November: the through 'Ashizuri' service, which had become a rapid train after conversion, is restored to limited-express status.
• 19931 October: Saga-Kōen Station opens as a new intermediate station.
• 19971 October: the Tosa Kuroshio Railway Sukumo Line opens as a continuation of the Nakamura Line; the maximum speed of limited expresses within the Nakamura Line is raised to 110 km/h.
• 200322 April: Uminoōmukae Station opens. 8 December: a large landslide between Kaina and Iyoki suspends services there until 10 January 2004.
• 201526 September: the Nakamura and Sukumo lines are jointly given the public nickname 'Shimanto Kuroshio Line'.
• 202014 March: through-running of the 'Nanpū' limited express from Okayama onto the line is discontinued in the timetable revision.
• 20232 June: a derailment caused by torrential rain leads to a suspension; full service is restored on 10 June 2023.
• 202416 March: the timetable revision cuts the line's limited expresses to eight round trips (one 'Shimanto' and seven 'Ashizuri'), of which 1.5 round trips continue onto the Sukumo Line.