The 23-kilometer-long old Mountain Line was built in 1908, early in the Japanese colonial era. Trains on this line began by winding through the precipices of Sanyi Rural Township, and from there cut through mountains and crossed rivers. This was the last north-south line in western Taiwan to be completed, and the most difficult stretch of track from an engineering point of view, with nine tunnels and three bridges.
全長約二十三公里的舊山線鐵路建於日據初期（西元一九○八年），行經三義鄉境內崎嶇蜿蜒的山路，穿越重山巨溪，是台灣西部縱貫線最後完成、工程最艱鉅的路段。