The Garabit viaduct in the Cantal region is Gustave Eiffel's greatest achievement. It was the beginning of his sophisticated projects for the Eiffel Tower but also triggered his interest in meteorology and aerodynamics. The region is mountainous and subject to high winds. The viaduct therefore required technical prowess never achieved before despite the construction of the viaduct over the Duro, in Porto, Portugal in 1876. The stability of the box girders, which served as the main rafters, and the structure's resistance to wind were the main challenges of the project. The engineers in charge of the construction, Boyer and Baudry, called on Gustave Eiffel because of his experience in this field.
It was a novel project because of the great height that separated the railway line from the river, twice that of the viaduct at Porto, 122 metres instead of 61 metres. Eiffel commented: "Suffice to say it is higher than the towers of Notre Dame in Paris with the Vendôme column on top". This comparison of the height of the buildings is demonstrated in the drawing, along with precise technical indications: the plan of the viaduct, the plans of the pillars and their elevations and cross-sections. Also visible are two masonry structures which start the viaduct on either side of the valley. The arched design had already been tested on the Duro. The viaduct took two full years to build, plus one year to develop the surrounding area in order to house and feed the workers. The viaduct was also unusual because the rails were lower than the deck so that any carriages blown off the tracks could be recovered. The drawing is touching because it shows three prestigious architectural constructions from different periods.