[1] After the Chosen Synthetic Oil Company opened a large factory in Aoji-ri (now Haksong-ri) in 1937[2] to produce synthetic oil from the bituminous coal mined in the area,[3] the Chosen Coal Industry Company built a railway line, called the Ao Line, to connect its mines to the chemical factory and to the South Manchuria Railway's North Chosen East Line, opening the first 5.9 km (3.7 mi) section from Aoji to Hoeam for passenger and freight service on 9 September 1938.

[4] The line was then extended, with a new 4.5 km (2.8 mi) section from Hoeam to Sinaoji (now called Ŭndŏk) and Obong opened on 14 September 1942.

[5] Following the partition of Korea, the entirety of the Ao Line was located in the Soviet zone of occupation.

The Provisional People’s Committee for North Korea nationalised all railways in the northern half of the country on 10 August 1946, and following the establishment of North Korea, the Korean State Railway was created.

A yellow background in the "Distance" box indicates that section of the line is not electrified.