Tren a las Nubes
Tren a las Nubes information
Lonely Planet review
From Salta, the Tren a las Nubes (Train to the Clouds) ascends the multicolored Quebrada del Toro, continuing past the important ruins of Tastil. The track makes countless switchbacks and even spirals, passing through 21 tunnels more than 3000m (9842ft) in total length, and crossing 31 iron bridges and 13 viaducts.
The trip's highlight is a stunning viaduct which is 64m (210ft) high and 224m (735ft) long, and spans an enormous desert canyon at La Polvorilla - a magnificent engineering achievement unjustified on any reasonable economic grounds. At Abra Chorillos, an altitude of 4575m (15,010ft) makes this the fourth highest operating line in the world.