Izuhakone Railway Sunzu Line

The line was not built by the state but by a private promoter. It opened for operations on 20 May 1898, when the Mame-Ai Railway (豆相鉄道) connected Zuso-Mishima Station — present-day Mishima-Tamachi — with Nanjō Station, the site of today's Izu-Nagaoka. A short extension on 15 June 1898 linked Mishima-machi to the original Mishima Station, and the following summer, on 17 July 1899, the line was pushed south from Nanjō to Daini (Ōhito), establishing most of the present route.

Ownership then changed hands several times in the early twentieth century. The line became part of the Izu Railway on 19 July 1907, which in turn became the Sunzu Electric Railway on 1 April 1912. That company merged with Fuji Hydroelectric in 1916 and was spun back out as the Sunzu Railway on 5 November 1917 — the corporate ancestor whose name the line still carries today.

Electrification came in stages during the 1910s. The Mishima-machi–Ōba section was electrified on 10 August 1918, through-running with the local tramway began in May 1919, and the line was fully electrified by June 1919. The southern terminus was then extended from Daini to Shuzenji on 1 August 1924, giving the Sunzu Line the Mishima–Shuzenji form it retains. Through-services to Tokyo over the connecting national railway began on a seasonal basis as early as 1933.

The modern operator emerged after the Second World War. On 1 June 1957 the Sunzu Railway was reorganised as the Izuhakone Railway, the company that runs the line today; the Izuhakone Railway is a member of the Seibu Group, with Seibu Railway as its parent company, and its corporate roots reach back to the Zusō Railway (豆相鉄道) founded in 1893. Under the new operator the line was modernised: the supply voltage was raised from 600 V to 1,500 V on 7 September 1959, automatic block signalling was extended across the whole Mishima–Shuzenji route by 1967, and all freight operations were discontinued on 16 June 1972.

The Sunzu Line's signature service began in the 1980s. The JR limited express "Odoriko" started running on 1 October 1981, through-operating from Tokyo over the Tōkaidō Main Line and onto the Sunzu Line to terminate at Shuzenji, and it remains the line's flagship train; from 13 March 2021 the Odoriko services were re-equipped with E257 series stock and a dedicated fare structure was set for the Sunzu Line portion. The railway also introduced its own rolling stock over the same period, with 3000 series trains entering service from 1979 and 7000 series trains from 1991.

Today the Sunzu Line functions as the principal public-transport spine of the central Izu Peninsula, linking the Tōkaidō Shinkansen and JR interchange at Mishima with the hot-spring resorts around Izu-Nagaoka and Shuzenji. Local services are supplemented by the through "Odoriko" expresses from the capital, and the line continues to modernise, introducing one-person operation on local trains from 2009 and contactless tap payment at all its stations in December 2024.

• 189820 May: the Mame-Ai Railway (豆相鉄道) opens the line, connecting Zuso-Mishima Station (now Mishima-Tamachi) with Nanjō Station (now Izu-Nagaoka).
• 189917 July: the line is extended south from Nanjō to Daini (Ōhito).
• 190719 July: the line becomes part of the Izu Railway.
• 19121 April: the operator becomes the Sunzu Electric Railway.
• 19175 November: after a 1916 merger with Fuji Hydroelectric, the railway is spun back out as the Sunzu Railway.
• 191810 August: the Mishima-machi–Ōba section is electrified.
• 1919The line is fully electrified by June; through-running with the connecting tramway begins in May.
• 19241 August: the southern terminus is extended from Daini to Shuzenji, giving the line its present Mishima–Shuzenji form.
• 19336 May: seasonal through-services from Tokyo begin running over the line on weekends.
• 19571 June: the Sunzu Railway is reorganised as the Izuhakone Railway, a Seibu Group company (parent: Seibu Railway), which still operates the line today.
• 19597 September: the supply voltage is raised from 600 V to 1,500 V.
• 1967Automatic block signalling is completed across the whole Mishima–Shuzenji route (25 March).
• 197216 June: all freight operations are discontinued.
• 19811 October: the JR limited express 'Odoriko' begins running, through-operating from Tokyo via the Tōkaidō Main Line to terminate at Shuzenji.
• 202113 March: the Odoriko services are re-equipped with E257 series stock and a dedicated fare structure is set for the Sunzu Line portion.
• 202418 December: contactless tap payment by card or smartphone begins at all stations on the line.