The Fréjus Rail Tunnel (also called Mont Cenis Tunnel) is a rail tunnel of 13.7 km (8.5 mi) length in the European Alps, carrying the Turin–Modane railway through Mont Cenis to an end-on connection with the Culoz–Modane railway and linking Bardonecchia in Italy to Modane in France. Its mean altitude is 1,123 m and it passes beneath the Pointe du Fréjus (2,932 m) and the Col du Fréjus (2,542 m).
Headed by the Italian civil engineer Germain Sommeiller, construction of the tunnel commenced during August 1857. From the onset, the tunnel was an ambitious engineering challenge, its gallery being twice the length of any tunnel previously constructed. Some figures believed that it would take as many as 40 years to complete; the total construction time was 13 years, the work having been greatly accelerated by the introduction of new technologies such as pneumatic drilling machines and dynamite. On 17 September 1871, the Fréjus Tunnel was opened to traffic for the first time, facilitating a new era of interaction between France and Italy.
The Fréjus tunnel remains an important link in the connection between Rome and Paris, via Turin and Chambéry. Following the development of car and truck transportation, the Fréjus Road Tunnel was built along the same path from 1974 to 1980. During the 2000s, the Fréjus Rail Tunnel underwent a series of works to modernise and improve it, including the increase of its bore to accommodate wider rail vehicles, including container trucks on piggy-back wagons, as part of the Autoroute Ferroviaire Alpine. A future high-speed rail tunnel to improve transit capacity between France and Italy, called the Mont d’Ambin Base Tunnel, is being planned as part of the Turin–Lyon high-speed railway project.