La Fregeneda-Barca D’Alba
We must have a huge difference in defining distance with Eric. Did it worth? Of course. Beautiful nature and the view around the route, the tunnels and the bridges, a lot of prickly pears (higo chumbo in Spanish, we have eaten some and took some with us), changing countries with a few steps and some nostalgy seeing all that infrastructure once used to be at the service for many passengers. Once you are at the Barca D’Alba station, you could see the remaining of the station, the waiting and VIP saloons, cantinas, customs office and a big building for repairing the cars. In a second, it makes you visualize the moment in black and white, the train coming and the families running around, the sound of the whistle.
According to Wikipedia, the route was used from its opening in 1887 until its closure on the Spanish side in 1985 . Recently they have tried to renovate to make it a tourism destination but it seems that the attempt was not successful 
Although you see a sign saying that it is not allowed to walk on the railway and the tunnels, no one seems to agree with it, including us.
The length of the route is around 18 km and can be walked in around 6 hours, you can find more details in the referenced link 
It is a lovely route and definitely suggested. There are cruisers coming from Porto to Barca D’Alba on the Duero River, next time we have planned to do it.