Shinonoi Line

The line was built outward from Shinonoi in stages around the turn of the twentieth century. The first segment, from Shinonoi to Nishijō (about 28.65 km), opened on 1 November 1900, bringing the stations of Inariyama, Obasute, Omi (today's Hijirikōgen) and Nishijō into service. The route was extended south from Nishijō to Matsumoto (about 24.58 km) on 15 June 1902, adding Akashina, Tazawa and Matsumoto, and then completed through to Shiojiri (about 13.84 km) on 15 December 1902, opening Murai and Shiojiri stations.

For its first years the line formed part of a longer corridor toward Tokyo. With the opening of the Okaya–Shiojiri section on 11 June 1906 the whole route from Hachiōji to Shinonoi was joined up, and when the government's line-naming ordinance was issued on 12 October 1909 the stretch from Shōheibashi to Shinonoi was designated the Chūō East Line. After the Chūō Main Line was completed throughout, the Shiojiri–Shinonoi portion was separated out on 1 May 1911 and given its present name, the Shinonoi Line.

The central section over the mountains was always the line's hardest stretch. Trains climb to the line's highest point, the 676-metre Kamuriki Station, through the 2,656-metre Kamuriki Tunnel, a place the railway regarded as a formidable obstacle in the age of steam. To haul trains over the grades the line came to rely on switchbacks; at its peak it had switchbacks at one station and three signal boxes, though rationalisation has since reduced these to two locations, one station and one signal box. The single station switchback that survives is the celebrated one at Obasute.

Modernisation came in the 1960s and early 1970s. Double-tracking advanced section by section from 1961, beginning with Shiojiri–Hirooka on 29 September 1961 and continuing through the Matsumoto area and on to the Tazawa–Akashina section by December 1966. Electrification at 1,500 V DC followed the same pattern, reaching Matsumoto from Minami-Matsumoto on 1 October 1964 and Shiojiri on 20 May 1965. At the October 1968 "Yonsantō" timetable revision the Nagoya–Nagano "Shinano" was upgraded to a limited express using 181-series diesel cars, and in February 1970 the deployment of DD51 diesel locomotives ended steam working on the line, marked by a farewell run of double-headed D51 locomotives.

The line was fully electrified and re-signalled in the early 1970s. Centralised traffic control (CTC) was commissioned over the entire line on 1 February 1972, and electrification was completed when the Matsumoto–Shinonoi section was energised on 28 March 1973. A timetable revision on 10 July 1973, tied to the electrification of the Chūō West Line, introduced the 381-series tilting electric trains on the "Shinano," and on 1 April 1976 Omi Station was renamed Hijirikōgen. On 6 June 1981 a down local train derailed between Obasute and the Kuwanohara signal box, injuring five passengers, and service was restored about twenty-five hours later.

At the breakup and privatisation of Japanese National Railways on 1 April 1987 the Shinonoi Line passed to JR East, with Japan Freight Railway (JR Freight) holding running rights over the whole line as a Type II operator. The route geometry was tidied in 1988, when the Nishijō–Akashina section was realigned on 10 September and the Shiosawa signal box abolished. More recent years have brought incremental upgrades: one-man operation began on some trains in March 2013, IC card (Suica) service reached the Matsumoto–Shiojiri section, and from February 2025 all fifteen stations received station numbers using the prefix "SN," with Suica extended over the Shinonoi–Matsumoto section the following month so that the whole line became usable with the card.

• 19001 November: the first segment, Shinonoi–Nishijō (about 28.65 km), opens as the Shinonoi Line; Inariyama, Obasute, Omi (now Hijirikōgen) and Nishijō stations open.
• 190215 June: the line is extended from Nishijō to Matsumoto (about 24.58 km); Akashina, Tazawa and Matsumoto stations open.
• 190215 December: the Matsumoto–Shiojiri section (about 13.84 km) opens, completing the line through to Shiojiri; Murai and Shiojiri stations open.
• 190611 June: the Okaya–Shiojiri section opens, joining up the whole route from Hachiōji to Shinonoi.
• 190912 October: the government line-naming ordinance designates the Shōheibashi–Shinonoi stretch the Chūō East Line.
• 19111 May: with the Chūō Main Line completed throughout, the Shiojiri–Shinonoi portion is separated out and named the Shinonoi Line.
• 19641 October: the Minami-Matsumoto–Matsumoto section is electrified at 1,500 V DC, beginning electrification of the line.
• 196520 May: electrification reaches Shiojiri (Shiojiri–Minami-Matsumoto section energised).
• 196610 December: the Tazawa–Akashina section is double-tracked, completing the 1961–1966 double-tracking programme on the line's lower sections.
• 19681 October: at the 'Yonsantō' timetable revision the Nagoya–Nagano 'Shinano' is upgraded to a limited express using 181-series diesel cars.
• 197022 February: DD51 diesel locomotives are deployed and steam working ends, marked by a farewell run of double-headed D51 locomotives.
• 19721 February: centralised traffic control (CTC) is commissioned over the entire line.
• 197328 March: the Matsumoto–Shinonoi section is electrified, completing electrification of the whole line; on 10 July the 381-series tilting EMU enters service on the 'Shinano'.
• 19761 April: Omi Station is renamed Hijirikōgen.
• 19816 June: a down local train derails between Obasute and the Kuwanohara signal box, injuring five passengers; service is restored about 25 hours later.
• 19871 April: at the breakup and privatisation of Japanese National Railways, the line passes to JR East; JR Freight becomes a Type II operator over the whole line.
• 198810 September: the Nishijō–Akashina section is realigned and the Shiosawa signal box is abolished.
• 2025February: all fifteen stations receive station numbers using the prefix 'SN'; in March Suica is extended over the Shinonoi–Matsumoto section so the whole line becomes usable with the card.