The ride up the Douro from Porto São Bento is recognized as one of the most beautiful train journeys in the world. Unlike the road the train hugs the river from around Aregos (a little over an hour from Porto) right up to Pocinho, the train’s terminus. The landscape, classified by UNESCO, is increasingly magnificent, especially after Pinhão, with the vineyards (Port and Douro reds) playing a starring role. Sadly many of the windows of the Douro train are engrained with dirt, spoiling both the landscape and any photos you take.
It is true that train ticket prices are low but they are throughout Portugal and the trains are often clean. I understand that the Douro line is under pressure due to dwindling traffic because of the move away from the country to towns. Presumably improved road links have also played their part as have the popularity of Douro River cruises.
I presume that trains running during the winter between Regua and Pocincho will carry very little traffic. It is a pity that Portuguese Railways and the Portuguese tourist bodies do not do more to promote the regular services on the line, although they do run special trains between Regua and Tua. The journey up the Douro has obvious appeal to wine lovers but is surely much wider than that as the success of the cruises shows.