明治39年(1906)完工之伯公坑至葫蘆墩輕便鐵道 (大安溪左岸)舊照Roughly translated, that's the Bogongkeng of Huludun Light Railroad, completed in 1906, on the left (north) bank of the Da-an River, which would put it in present-day Miaoli. Huludun is the old name of Fengyuan, which in those days covered a much bigger administrative area. Although you often hear that in Mandarin a 坑 is a pit, in Taiwanese it often designates a ravine filled by a creek.
The Chinese Wiki entry is here. This line connected the Da-an River with Sanchahe (三叉河), high above the Da-an River near what is now Sanyi. It carried construction materials for the rail line, as the picture shows. According to Wiki, by 1904 the northern end of the rail line had reached Sanchahe near Sanyi, and the southern end had reached Changhua. However, between them lay the 300 meter descent to the Da-an River (a 3.75% grade according to the county government site on this), which, along with the Dajia River, had to be bridged, and tunnels punched through the bluffs that separated them. This was a stopgap until all that engineering could be carried out. In 1908 the line was completed and a few months later the light rail line was dismantled. No trace is left of it, according to the county website.
Sanchahe is the red marker). I suspect this circular section of the rail line must have been located in what is now the gravel processing site just east of the 1 at the base of the mountain, and then it ran along what is now the 1 to Sanyi. There is a Bogongkeng (伯公坑) on the west side where you can hike to the top of that enormous jagged sandpile mountain in the lower west corner of this picture, but on the east side a 伯公溪 (Bogong River/Creek) is marked. Because that mountain of sand is unstable and a frequent site of slides, several of which are visible in the image above, I suspect it must originally have been built on the other side (east side) of what is now the highway.
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