I think I mentioned previously that as we headed east(ish) from Morbegno to Sondrio we were tracking along the route of the Addo River which feeds in to Lake Como. At the beginning we were to the south (right) of the river. However at about this point, another river (shown on the maps as Valle Di Tartano, though I believe that's the name of the valley rather than the water) flows in from some mountains to the south to join it. The delta of that join is fairly wide so the rail line swings north and over the river before its tributary joins it. That's what's happening here. For the rest of this leg of the trip we stayed to the north of the river.
This isn't exactly a shot of thundering torrents of alpine water, even after the other river joins it. It may be a different story in early spring after the snows melt.
Mental note: some day I want to see for myself.
If this was shot from a building rather than a moving train... I think perhaps that this is a view I would not tire of easily.