High in the Argentine Andes, the Tren a las Nubes climbs from Salta toward the Chilean border, gaining altitude through switchbacks, zigzags, and spiral tunnels carved into rock. The film follows the engineers and crews who keep the line running past 4,000 meters, where thin air, sudden storms, and unstable ground make every kilometer of track a hazard. Archival footage and present-day filming trace how the railway was built almost a century ago to move minerals and people through terrain that seemed to refuse a railway entirely, culminating at the La Polvorilla viaduct, a curved steel bridge suspended over a ravine. Interviews with rail workers and historians explain the construction techniques used to conquer the altitude and the ongoing maintenance needed to keep the line from being swallowed by landslides or washed-out embankments. The documentary treats the line as both an engineering feat and a working piece of Argentine infrastructure, still carrying tourists and freight through one of the most extreme rail environments on the continent.