Construction of the Cheongjin Line was accelerated under Mantetsu management, and by the end of 1917 it had been completed all the way to Hoeryeong, with the 13.4 km (8.3 mi) from Changpyeong to Pungsan opened on 16 September, and the remaining 24.7 km (15.3 mi) to Hoeryong opened on 25 November of that year.

[2] To access this, a new 14.9 km (9.3 mi) line between Nanam and Suseong (on the Cheongjin−Changpyeong line) was built, being opened on 10 December 1919; Gangdeok station, located 7.2 km (4.5 mi) from Nanam, was opened on 1 August 1922.

Construction of the southern portion of the Hamgyeong Line continued at the same time, with a new 69.5 km (43.2 mi) section from Yeongheung to Hamheung being opened on 15 December 1919.

[1] The final 26.1 km (16.2 mi) gap between Geosan and Gunseon was closed on 1 September 1928, completing the line in its entirety from Wonsan to Hoeryeong.

[1] These lines later played a major role in the Japanese invasion of Manchuria, and grew further in importance after the establishment of the puppet state of Manchukuo.

The privately owned Domun Railway was formed in 1920, opening its first line, (40.4 km (25.1 mi)) from Hoeryeong to Sangsambong on 5 January 1920.

[4] In order to create the shortest possible route from Japan to eastern Manchuria, Sentetsu began construction of a line from Unggi (now Sŏnbong) to Donggwanjin via Namyang in 1929.

The existing line, running via Gangdeok Station and the marshalling yard, was detached from the Hamgyeong Line and designated the Gangdeok Line; at the same time, a 2.4 km (1.5 mi) connection from Cheongjin Seohang Station (Cheongjin West Port) to Gangdeok was built, to allow southbound trains to access the marshalling yard without having to reverse at Nanam.