Sure, a train ride can be a pretty passive experience, but when it climbs from the subtropical city of Salta to the high-puna town of San Antonio de los Cobres and the extraordinary La Polvorilla viaduct, it's something special. There are actually two versions of the"Train to the Clouds": a comfortable but relatively expensive tourist excursion that runs from April to October only, and a cheaper freight train that's worth the effort and frustration that getting on it can entail. The tracks pass through colorful desert canyons to the high puna, around 12,000 feet above sea level, through routes taken by some of the earliest Spanish conquistadors from Peru. There are few train trips remaining in Argentina, and this is certainly the most scenic of them.