Flower Nagai Line

The railway was built to improve transport in the Okitama region of southern Yamagata, laid under the provisions of the Light Railway Act. It opened in stages from 1913 as the Nagai Light Railway Line (長井軽便線). The first segment, from Akayu to Ringō (6.9 km), opened on 26 October 1913, with stations at Miyauchi-machi and Ringō. The line was extended from Ringō to Nagai (11.4 km) on 15 November 1914, adding the stations at Nishi-Ōtsuka, Imaizumi, Tokiniwa and Nagai.

On 2 September 1922 the route was renamed the Nagai Line (長井線). Construction then continued northward: the Nagai–Ayukai segment (9.7 km) opened on 11 December 1922, and the final Ayukai–Arato segment (2.6 km) opened on 22 April 1923, completing the line through to Arato. The revised Railway Construction Act of 1922 had also planned an extension from Arato to Aterazawa on the Aterazawa Line (the so-called Sakō Line), but it was never built.

Under JNR the line settled into the role of a rural branch. Diesel multiple units replaced steam locomotives on 15 November 1954, and two infill stations were added in the area around Miyauchi — a new station opened on 1 June 1959 and Minami-Nagai opened on 20 May 1960. Freight services were progressively withdrawn, the Nagai–Arato freight ending in 1981 and the Imaizumi–Nagai freight in 1982.

With the passage of the JNR Reconstruction Act, the Nagai Line was designated a third-batch Specified Local Line, and its abolition was approved on 28 October 1986 as one of the lightly used routes marked for conversion. On 1 April 1987 the line passed from JNR to the East Japan Railway Company (JR East) in the privatisation of the national railways, and what remained of its freight service ended at the same time.

The transfer to a third-sector operator followed soon after. On 25 October 1988 the JNR-era Nagai Line was abolished and handed over to the newly established Yamagata Railway, which reopened it the same day as the Flower Nagai Line. The new operator opened Nanyō-Shiyakusho Station, renamed Miyauchi-machi to Miyauchi and Nishi-Miyauchi to Orihata, and over the following years added further stations — Shirousagi in 1989, Ayame-Kōen in 2002, and Shikinosato in 2007. Centralised traffic control was introduced on 23 October 1997.

Under Yamagata Railway the line has leaned on its scenery and local character to survive declining ridership. Service is run by single-car YR-880 diesel railcars as one-person-operated all-stations trains shuttling along the line, with the Akayu–Arato run taking about an hour; at Imaizumi the line meets JR East's Yonesaka Line, and at Akayu it connects with the Yamagata Line and the Yamagata Shinkansen. The line has appeared in popular culture, including the 2004 film Swing Girls, and the route's old double-Warren truss bridge over the Mogami River near Arato was recognised as a Selected Civil Engineering Heritage in 2008.

• 191326 October: the Akayu–Ringō segment (6.9 km) opens as the Nagai Light Railway Line, with stations at Miyauchi-machi and Ringō.
• 191415 November: the Ringō–Nagai segment (11.4 km) opens, adding Nishi-Ōtsuka, Imaizumi, Tokiniwa and Nagai stations.
• 19222 September: the route is renamed the Nagai Line. 11 December: the Nagai–Ayukai segment (9.7 km) opens, adding Uzen-Narita, Koguwa and Ayukai stations.
• 192322 April: the Ayukai–Arato segment (2.6 km) opens, completing the line through to Arato.
• 195415 November: diesel multiple units replace steam locomotives on the line.
• 19591 June: a new station opens in the Miyauchi area (Nishi-Miyauchi, later renamed Orihata).
• 196020 May: Minami-Nagai Station opens.
• 1982Freight service is withdrawn in stages, the Nagai–Arato freight ending in 1981 and the Imaizumi–Nagai freight on 15 November 1982.
• 198628 October: the Nagai Line's abolition is approved, designated a third-batch Specified Local Line for conversion.
• 19871 April: in the privatisation of JNR, the line passes to the East Japan Railway Company (JR East); its remaining freight service ends.
• 198825 October: the JNR-era Nagai Line is abolished and transferred to Yamagata Railway, reopening the same day as the Flower Nagai Line; Nanyō-Shiyakusho Station opens and Miyauchi-machi and Nishi-Miyauchi are renamed Miyauchi and Orihata.
• 198916 December: Shirousagi Station opens.
• 199723 October: centralised traffic control (CTC) is introduced (Miyauchi–Nagai special automatic block signalling completed at the same time).
• 20029 June: Ayame-Kōen Station opens.
• 200713 October: Shikinosato Station opens.
• 2008The Mogami River bridge near Arato, a Meiji-era double-Warren truss bridge, is selected as a Selected Civil Engineering Heritage.