Tren a las Nubes, Mountain tourist train in Salta Province, Argentina

Tren a las Nubes is a tourist railway service operating through the northern Andes of Argentina's Salta Province. The route crosses twenty-nine bridges, passes through twenty-one tunnels, and travels over thirteen viaducts in one of the highest regions of South America.

Studies for a railway across the Andes began in 1889, and construction started in 1921 to link northern Argentina with Chile. American engineer Richard Maury designed the route without rack-and-pinion traction, using zigzags and spirals. The La Polvorilla viaduct was finished on 7 November 1932, and the complete line was inaugurated on 20 February 1948 after delays and a two-year halt in work.

The train's name refers to the extreme altitude of the route through the Andes and the low-hanging clouds often seen along the way. Passengers hear traditional folk music during the journey and can sample regional dishes from the highland provinces.

Since 2015 the train itself boards at San Antonio de los Cobres station, reached by bus from Salta along National Route 51 through Campo Quijano and El Alfarcito. The rail leg to La Polvorilla viaduct takes about an hour, with 30 minutes at the viaduct before returning. A medical post with medicinal oxygen travels on board; an extra locomotive accompanies each trip for emergencies.

The La Polvorilla viaduct, the highest on the line, was finished on 7 November 1932 and is 224 m (735 ft) long and 64 m (210 ft) high. The train crosses it at 4,220 m above sea level, the fifth highest railway point in the world, then pauses about 30 minutes before returning to San Antonio de los Cobres.