[ The Route of the Tunnels ]
Once a railroad that connected Oporto to Paris, this way doesn't see a train apparently since 1985.
This route from "la Fregeneda" (Spain) to Barca d' Alva (Portugal) is presently a trail for those who not only don't fear a 17 km hike but also are willing to challenge their fear of heights.
La Fregeneda (Spain) | Barca d' Alva (Portugal)
2014