After the Chosen Synthetic Oil Company opened a large factory in Aoji-ri (now Haksong-ri) in 1937 Aoji-ri Chemical Complex to produce synthetic oil from the bituminous coal mined in the area, 북한 아오지 탄광과 석유화학공업(1편) 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 kilometers section from Aoji to Hoeam for passenger and freight service on 9 September 1938.