The Tren A Las Nubes Train, Salta Province, Argentina

The Tren a las Nubes or Tren de las Nubes (English: Train to/of the Clouds) is a touristic train service in Salta Province, Argentina. The service runs along the eastern part of the C-14 line of the Ferrocarril General Manuel Belgrano, that connects the Argentine Northwest with the Chilean border in the Andes mountain range, over 4,220 metres (13,850 ft) above mean sea level, the fifth highest railway in the world. Originally built for economic and social reasons, it is now primarily of interest to tourists as a heritage railway.
The railway line has 29 bridges, 21 tunnels, 13 viaducts, 2 spirals and 2 zigzags. Because of the design decision not to use a rack-and-pinion for traction, the route had to be designed to avoid steep grades. The zigzags allow the train to climb up driving back and forth parallel to the slope of the mountain.