This is the shortest of all Japan Railways Group lines named Main Line. For comparison, the second shortest is Rumoi Main Line, although it once ran a longer distance. The line is divided into three sections. Each section has an alias. ;Wakamatsu Line: Wakamatsu–Orio, 10.8 km ;Fukuhoku Yutaka Line: Orio–Keisen, 34.5 km ;Haruda Line: Keisen–Haruda, 20.8 km
The 39 km Wakamatsu to Iizuka section was built by the Chikuho Kogyo Railway, opening between 1891 and 1893, with the 12 km extension to Chikuzen Uchino opening in 1895. The company was nationalised in 1907, with the 15 km Chikuzen Uchino to Harada section opening in 1929. The 25 km section from Wakamatsu to Nogata was double-tracked between 1893 and 1896, with the 4 km Nogata to Kotake section double-tracked between 1904 and 1906. The 8 km Kotake to Iizuka section was double-tracked between 1942 and 1944. The 35 km Orio to Keisen section was electrified in 2001.
Former connecting/transfer lines
Orio Station: the Nishitetsu Kitakyushu Line, a gauge line electrified at 600 V DC was opened from Orio - Moji between 1911 and 1914. It connected to a 10 km loop line opened between 1912 and 1929, and closed in 1985. The Moji - Sunatsu section closed in 1985, the Sunatsu - Kurosaki section closed in 1992, and the remainder closed in 2000.
Nakama Station: the 3.5 km Katsuki Line opened in 1908, with passenger services being introduced in 1912. Freight services ceased in 1974, and the line closed in 1985. A 3.8 km line from Katsuki hauled coal from 1915 until the mine closed in 1954.
Nogata Station: A 14 km gauge line to Fukumaru operated from 1914 until 1938.
Katsuno Station: the 5 km Miyata Line opened in 1902. Freight services ceased in 1982, and the line closed in 1989. A 2 km branch to Sugamuta served the Kajima coal mine from 1904 until 1977.
Kotake Station: the 8 km Kōbukuro Line to Futase opened in 1894 as a horse-drawn line servicing the Tetsuji mine. Steam locomotion was introduced in 1912, and passenger services in 1913. The line closed in 1969.
Iizuka Station: the 26 km Kamiyamada Line opened to Shimoyamada in 1898, extended to Kamiyamada in 1929, and to Buzenkawasaki in 1966. Freight services ceased in 1980, and the line closed in 1988. A 2 km gauge line from Okuma - Okumamachi operated from 1924 until 1933.
