Katsuki Line was a very short local line located in Chikuhou area in North Kyuusyuu. It branched from Chikuhou Main Line at Nakama station and had only three stations. Chikuhou area was known as coal mining area and Katsuki Line was built to transport the coal. By mid 1970's all mines around the line were closed down and Katsuki Line remained only as a small local line.
I used to go to Kyuusyuu by night train from Tokyo and early in next arrived at North Kyuusyuu and always took photos of morning commuter train (only one steam train on Katsuki Line then) hauled by JNR 8620 class.
JNR 8620 Class
8620 class was the firstly mass-produced passenger locos in Japan from 1914 to 1929 and very successful. They served much longer than their successor C50 class. Usually passenger locos have two axle leading truck to cope with high-speed cornering, but 8620 class used first driving wheels as leading truck wheels. The leading wheel axle and first driving wheel axle were connected together with specially designed sub frame. Actually I saw the drawing of 8620 class, but could not understand how the mechanism worked.
Many of them served for local passenger and freight trains until the end of JNR steam era.