Aterazawa Line

The line was built by the state as a light railway. Its first section, the 12.9-kilometre stretch from Yamagata to Uzen-Nagasaki, opened on 20 July 1921 as the Aterazawa Light Line (左沢軽便線). It was extended a short distance from Uzen-Nagasaki to Sagae, a further 4.4 kilometres, on 11 December of the same year, reaching the largest town along the route.

The remaining 8.9-kilometre section from Sagae onward to Aterazawa opened on 23 April 1922, completing the through route from Yamagata to Aterazawa. On 2 September 1922 the line was reclassified and renamed simply the Aterazawa Line. In 1927 long-distance Ōu Main Line trains began stopping at Kita-Yamagata, strengthening the junction where the branch met the trunk route.

The early decades saw the line settle into its role as a rural feeder. On 25 December 1951 five new stations were opened along the route in a single day, doubling the number of stations and bringing the railway closer to the communities it served. Steam haulage gave way to diesel railcars from 1 April 1958, modernising local services and cutting journey times.

In 1982 the line was modernised for more efficient operation: centralised traffic control (CTC) was introduced in March, with the control centre placed at Kita-Yamagata, and rapid services were introduced at the same time. Later that year, on 5 November 1982, freight operations over the whole line were abolished, leaving it a purely passenger railway. On 1 April 1987 the line passed, with the rest of the national network, from the Japanese National Railways to the newly formed JR East under the division and privatisation of JNR.

Under JR East the line was streamlined further. One-man (driver-only) operation began across the whole line on 10 March 1990, and a dedicated Aterazawa Line operating office was established. On 2 July 1998 the official line section was redefined as running from Kita-Yamagata to Aterazawa, formalising the long-standing practice of treating the Yamagata–Kita-Yamagata segment as part of the Ōu Main Line even though Aterazawa Line trains continued to run through to Yamagata.

The line remains a single-track local artery for the Mogami River valley. Its bowstring-truss bridge over the Mogami River, the Mogami-gawa Kyōryō, was recognised in 2008 as a Civil Engineering Heritage by the Japan Society of Civil Engineers. Services were suspended for a time after the Tōhoku earthquake of 11 March 2011 before being restored. In recent years JR East has continued to integrate the line into its wider network, introducing Suica IC-card acceptance on the Kita-Yamagata–Sagae section on 16 March 2024. Today the Aterazawa Line is worked by Kiha 101 diesel railcars and serves chiefly local passengers between Yamagata, Sagae and Aterazawa.

• 192120 July: the first section, Yamagata–Uzen-Nagasaki (12.9 km), opens as the Aterazawa Light Line (左沢軽便線).
• 192111 December: the line is extended from Uzen-Nagasaki to Sagae (4.4 km).
• 192223 April: the Sagae–Aterazawa section (8.9 km) opens, completing the through route from Yamagata to Aterazawa.
• 19222 September: the Yamagata–Aterazawa route is reclassified and renamed the Aterazawa Line.
• 192711 September: Ōu Main Line trains begin stopping at Kita-Yamagata Station.
• 195125 December: five new stations open in a single day, doubling the number of stations on the line.
• 19581 April: services are converted to diesel railcar operation.
• 1982March: centralised traffic control (CTC) is commissioned, with the control centre at Kita-Yamagata, and rapid services are introduced.
• 19825 November: freight operations over the whole line are abolished, leaving it a passenger-only railway.
• 19871 April: under the division and privatisation of JNR, the line passes to the newly formed JR East.
• 199010 March: one-man (driver-only) operation begins across the whole line; a dedicated Aterazawa Line operating office is established.
• 19982 July: the official line section is redefined as Kita-Yamagata–Aterazawa.
• 2008The Mogami River Bridge (最上川橋梁) is selected as a Civil Engineering Heritage by the Japan Society of Civil Engineers.
• 201111 March: the line is suspended throughout following the 2011 Tōhoku earthquake, before later being restored.
• 202416 March: Suica IC-card service begins on the Kita-Yamagata–Sagae section.