Koumi Line

The line took its name from the village of Koumi, a place roughly midway along the route; the name ("little sea") is said to derive from a lake that once existed there after a collapse of Mount Tengu dammed the Chikuma River. The line came into being from two separate efforts — a private railway building north from Komoro and the state railway building south from Kobuchizawa — which were eventually joined into a single through route.

The southern half of what is now the line began as a private undertaking. The Saku Railway Company opened its first section, from Komoro to Nakagomi, on 8 August 1915, operating as a light railway, and extended it to Haguroshita on 28 December that year. On 11 March 1919 the Saku Railway reached Koumi, completing its line between Komoro and Koumi.

The state railway then took up the work from the Chūō Main Line end. The Japanese Government Railways opened the Koumi–Saku-Uminokuchi section on 27 December 1932 — the segment that first carried the "Koumi Line" name — and on 27 July 1933 opened the Kobuchizawa–Kiyosato section as the "Koumi South Line."

On 1 September 1934 the government purchased and nationalised the Saku Railway's line between Komoro and Koumi, folding it into what was now called the "Koumi North Line." Construction across the high central gap continued: the Saku-Uminokuchi–Shinano-Kawakami extension opened on 16 January 1935.

The final gap was closed on 29 November 1935, when the Kiyosato–Shinano-Kawakami section opened and the line was completed end to end. With through running established, the Koumi South and Koumi North lines were merged and the unified route was renamed the Koumi Line.

The line was an early adopter of internal-combustion traction: the Saku Railway received authorisation to use petrol-powered cars on 12 November 1930 and began diesel-railcar operation on 20 December that year. It has remained non-electrified ever since. On 1 April 1987, with the division and privatisation of Japanese National Railways, the Koumi Line passed to the East Japan Railway Company.

Under JR East the line has been a showcase for new diesel technology. KiHa 110 series DMUs entered service on 8 November 1991, and from 31 July 2007 the line also ran the KiHa E200, a new vehicle that was the world's first to adopt a series-hybrid system. JR East has also leaned into the line's scenic, high-altitude character: in July 2017 it launched the sightseeing train "High Rail 1375," whose name refers to the 1,375 m elevation of the highest point on any JR line, which the train traverses.

• 19158 August: the Saku Railway Company opens its first section, Komoro–Nakagomi, operating as a light railway; on 28 December it extends to Haguroshita.
• 191911 March: the Saku Railway reaches Koumi, completing its Komoro–Koumi line.
• 1930The Saku Railway is authorised to use petrol-powered cars on 12 November and begins diesel-railcar operation on 20 December.
• 193227 December: the Japanese Government Railways open the Koumi–Saku-Uminokuchi section — the segment that first carries the 'Koumi Line' name.
• 193327 July: the Kobuchizawa–Kiyosato section opens as the 'Koumi South Line'.
• 19341 September: the government purchases and nationalises the Saku Railway's Komoro–Koumi line, incorporating it into the 'Koumi North Line'.
• 193516 January: the Saku-Uminokuchi–Shinano-Kawakami extension opens.
• 193529 November: the Kiyosato–Shinano-Kawakami section opens, completing the line end to end; the Koumi South and North lines are merged and renamed the Koumi Line.
• 19871 April: with the division and privatisation of Japanese National Railways, the line passes to the East Japan Railway Company (JR East).
• 19918 November: KiHa 110 series DMUs enter service on the line.
• 200731 July: the KiHa E200 enters service — the world's first vehicle to adopt a series-hybrid system.
• 2017July: JR East launches the sightseeing train 'High Rail 1375', named for the 1,375 m elevation of the highest point on any JR line, which it traverses.