La Polvorilla Viaduct is the name given to the best known of the viaducts through which the Tren a las Nubes travels on the C-14 branch of the General Belgrano Railway of the Argentine railway network. It is located at 24°12′9.6″S 66°24′52.6″W, in the province of Salta, about five kilometers east of the town of Chorrillos. A dozen kilometers to the northwest is San Antonio de los Cobres, a little after crossing the so-called Eastern Cordillera by a pass that separates it from the so-called Sierra de los Pastos Grandes.

The bridge is a structure of steel beams 223.5 meters long, with a maximum height of 63 meters above the ground (similar in height to the Obelisk of Buenos Aires) and weighing 1,590 tons, on a piece of land located at 4,200 meters above sea level (13,779 feet) —thus constituting one of the highest bridges and railway sections in the world^{[1 ]}u200b above sea level—. Since its inauguration it was considered a monumental work of engineering, becoming a tourist attraction. In the 1970s, the company Ferrocarriles Argentinos devised a tourist passenger service, the Tren a las Nubes, with a terminus just beyond La Polvorilla.

The builders of the branch, led by engineer Richard Maury, faced the problem of crossing a wide and deep ravine through which a tributary of the San Antonio de los Cobres River runs. To overcome the obstacle, it was necessary to make a detour of 18 kilometers to the north, in order not to lose the altitude gained on the way, or to build a long bridge. The latter was the chosen option, but it faced 3 additional difficulties: As the rails are on an rising ramp, the western abutment was 4.5 m higher than the eastern one; the section had to be curved and also banked. The chosen solution involved the construction of six sections of 14 meters in length and seven of 20 meters, supported on six steel piles with a stone base. The construction of the parts was entrusted to the shipyard Cantiere Navale Triestino of Monfalcone, Italy (today part of Fincantieri). Construction was completed in 1932, but it was inaugurated on November 7, 1939. During construction three workers lost their lives, whose bodies are buried in the Mina Concordia cemetery.