The tunnel was completed in 1924 as a single-track bore on the Yilan Line — the coastal railway that knit Taiwan's northeast together during the Japanese colonial period. Cutting through the ridge at Caoling, it allowed trains to bypass the headland where the coast becomes impassable cliffs, shaving time off the journey between Taipei and Yilan. For more than six decades it served that purpose without fanfare, carrying the rhythm of timetables and cargo through 2.167 kilometers of mountain. The engineering was practical, not grand — oil lamps for light, stone portal arches at each end, single gauge track down the center. When the parallel new tunnel opened in 1986, the old bore simply stopped. No ceremony, no closure event. The trains moved next door and the darkness moved in.

Abandoned infrastructure in Taiwan does not always disappear. The Old Caoling Tunnel sat dormant through the late 1980s and 1990s while the northeast coast developed as a scenic area. The Northeast Coast and Yilan National Scenic Area Administration eventually recognized what lay in the hillside: a ready-made corridor, already bored, already proven, needing a new purpose. Converting the old tunnel into a bikeway required lighting the interior and resurfacing the floor — the original track-bed design was echoed in the cycling path, which was laid out in the style of a rail track as a quiet nod to origins. Oil lamps gave way to fluorescent strip lights, with low-energy bulbs in replica lanterns spaced along the passage. After years of work, the tunnel reopened to cyclists in August 2009.

Walking or cycling through 2.167 kilometers of tunnel is not a passive experience. The portals at each end frame daylight like painting borders; in between, the rock presses close and the air holds a different temperature than the hillside outside. The replica lanterns cast warm pools against the fluorescent wash, creating an atmosphere that is neither purely historical nor purely modern — something in between, where the memory of trains is half-present. Artworks at the entrance acknowledge the passage of time and purpose. The coastal setting makes the tunnel a natural waypoint on the Northeast Coast Cycleway, and riders who emerge from the Gongliao end look out toward the Pacific, blinking in the brightness after the long, lit dark.

The tunnel's geography is part of its meaning. It pierces the ridge that forms the spine of Taiwan's northeast corner, where volcanic hills drop sharply to a rocky coast lashed by the Pacific. On the Gongliao side, the land opens toward fishing communities and surf beaches. On the Fulong side, the scenery is gentler — the Shuangxi River valley and the sandy crescent of Fulong Beach, one of Taiwan's most popular. Cyclists who use the tunnel typically combine it with the coastal cycle route, making a loop that takes in the tunnel's interior drama and the ocean's exterior expanse. The contrast — dark corridor, bright sea — is the experience's defining rhythm.

Most infrastructure that goes dark in the twentieth century stays dark. The Old Caoling Tunnel is a reminder that the reverse is possible — that a bore through a hillside can outlast its original purpose and find another. The railway that built it is gone from this stretch; the lanterns that once guided engineers are echoed in replicas that guide cyclists. The rock itself has not changed. A hundred years of trains and then a generation of bikes: the mountain accommodates both without complaint. For visitors arriving from Taipei, a short train ride to Fulong and then a pedal into the ridge, the tunnel offers something that coastal Taiwan provides only rarely — the sensation of going through rather than around, of understanding a landscape from its interior.