The Mitsui Electric Tramway (三井電気軌道, Mitsui Denki Kidō) opened a 1,435 mm (4 ft 8+1⁄2 in) gauge line electrified at 600 V DC from Miyanojin to Kitano in 1915. The Kitano to Amagi section opened in 1921.[citation needed]
On 30 June 1924, the Mitsui Electric tramway was absorbed into the Kyushu Railway system, this line becoming the Mitsui Line. In 1942, the Kyushu Railway merged with the Kyushu Electric Railway, becoming the Nishi-Nippon (translates as West Japan) Railway.
In 1948, the line voltage was increased to 1,500 V DC, the same year through-running commenced with the Tenjin Ōmuta Line and the line was renamed the Amagi Line.[citation needed]