Garabit Viaduct

Gustave Eiffel's engineering firm built the Garabit Viaduct between 1882 and 1884 to carry the railway from Marvejols to Neussargues across the Truyère gorge in the Cantal, on a single track 122 metres above the river. The structure is a two-hinged parabolic arch of latticed wrought iron, its longest span 165 metres, that was for a time the highest bridge in the world; the deck rides on iron piers rising from the arch's crown rather than resting directly on the arch itself, letting the whole structure flex under load and temperature. Chief engineer Maurice Koechlin, who had worked out the calculations for what became the Eiffel Tower five years later, is credited with the design; Garabit is often read as a proof of the same wind-load and lattice principles the Tower would apply at a different scale. Opened to traffic in 1885, the line still carries occasional passenger trains, though cracks discovered in a foundation pier in 2009 led to a lasting reduced speed limit of 10 km/h across the crossing. It is a classified monument historique.