In Japan, third-sector railways (第三セクター鉄道, dai-san sekutā tetsudō), sometimes translated as semi-public sector railways, are railways operated by a joint corporation that has been invested in by both public and private sectors, and which maintain a public–private partnership.^[1] These lines are most often operated by small companies owned by both a prefectural or municipal government and smaller private interests, who both invest in and manage the line.^[2][3] Third-sector lines are generally former lines of the JR Group – or, before 1987, Japanese National Railways (JNR) – that have been divested from the companies.^[4]

Examples of lines that were proposed for abolishment or transfer to third-sector companies throughout the 20th century include the Deficit 83 Lines and specified local lines.

Public, government-operated enterprises such as the former Japanese National Railways are considered to be the "first sector," while private sector enterprises serve as the "second sector." Thus, enterprises that fall into neither of these first two categories are said to be in the "third sector."^[5]

Third-sector lines often begin operations on parallel conventional (local) lines when Shinkansen service is extended to a new area. In March 2024, the Hokuriku Shinkansen was extended south, its terminus changing from Kanazawa in Ishikawa Prefecture to that of Tsuruga in Fukui Prefecture. Like other sections of the Hokuriku Main Line, JR West gave up ownership of operation of the local line on this same route and transferred it to a newly formed company known as Hapi-Line Fukui. The same had occurred years prior with sections of the local line now owned by IR Ishikawa Railway, Ainokaze Toyama Railway, and Echigo TOKImeki Railway.

This same arrangement can be seen in the Tōhoku region, with portions of JR East's Tōhoku Main Line being transferred to Aoimori Railway and Iwate Galaxy Railway following the extension of the Tōhoku Shinkansen in throughout the early 2000s.

Other reasons for establishment include takeovers of unprofitable private railway lines that require additional investment from local governments, and new transportation [ja] systems such as automated guideway transit, monorails, or maglev systems built in areas of rapid development and increasing urban density.^[6]

As local governments with close contacts to communities and private companies with experience running rail infrastructure are jointly invested in these lines, there is an elevated level of flexibility in management and adjustment of operations, which can be done according to frequently changing circumstances and needs of local communities. These lines, however, are most often unprofitable, require funding via taxes, and may still run the risk of facing abolishment due to low ridership.^[7][5]

• List of Japanese third-sector railway lines