The Tren A Las Nubes Train, Salta Province, Argentina

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.
History
The construction of the railway started in 1921, to connect the North of Argentina with Chile across the Andes, and to serve the borax mines of the area. The viaduct La Polvorilla, the highest of the line, was finished on 7 November 1932. The complete railway was inaugurated on 20 February 1948, but it was not until the late 1970s that it started being visited by tourists. The route was designed by American engineer Richard Fontaine Maury, after whom one of the stations has been named.