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https://daitetsu.jp/eng/abt
The Abt rail system, developed in Switzerland in the 1880s, is a narrow gauge track that specializes in climbing inclines by using a 3rd track that is toothed.
The Oigawa Railway's Ikawa Line is the only Abt system in Japan.
Featured in anime series Yuru Camp Season 3, episodes 3 and 4.