The Turin–Lyon high-speed railway is an international rail line under construction between the cities of Turin and Lyon. It is intended to link the Italian and French high-speed rail networks and will be long. The core of the project is its international section, which will cross the Alps through the Mont d'Ambin Base Tunnel between the Susa Valley in Piedmont and Maurienne in Savoie. At , that tunnel will be the longest rail tunnel in the world, ahead of the Gotthard Base Tunnel. The estimated total cost of the line is €25 billion, of which €8 billion is for the international section. The latter is the only part of the line where work has started.
Like the Swiss NRLA project, the line has twin aims of transferring freight traffic across the Alps from trucks to rail to reduce CO2 emissions and local air pollution and of providing faster passenger transport to reduce air traffic. The new line will considerably shorten the journey times, and its reduced gradients and much wider curves compared to the existing line will also allow heavy freight trains to transit between the two countries at and with much reduced energy costs. In spite of the name often used by media, the line is not high-speed under the definition used by the European Commission: its design speed of is 12% below the threshold used by the commission to define high-speed railways. The line is therefore part instead of the TEN-T Trans-European conventional rail network within its "Mediterranean Corridor"—previously "Corridor 6". The European Union funds 40% of the tunnel costs, and has indicated its willingness to increase its contribution to 55%, as well as to help fund its French accesses if those go beyond mere adaptations of the existing infrastructure.
The project has been criticized for its cost, because traffic (both by motorway and by rail) was decreasing when the project was decided, for potential environmental risks during the construction of the tunnel, and because airplanes will still, after including time to and from the airport and through security, be slightly faster over the full Milan–Paris route. A 2012 report by the French Court of Audit questioned the realism of the costs estimates and traffic forecasts. Opposition to the project is mostly organised under the loose banner of the No TAV movement.
Civil engineering work started in 2002 with the construction of access points and geological reconnaissance tunneling. A gallery tunneled between 2016 and 2019 from Saint-Martin-de-la-Porte towards Italy was presented as reconnaissance work, but it was dug at the position of the south tube of the tunnel and at its final diameter. It effectively represents the first 8% of the final tunnel length. As of 2021, the expected completion date for the base tunnel was 2032.
Pre-construction studies
The merits of the new line were the subject of heated debate, primarily in Italy. After a 2005 attempt to start reconnaissance work near Susa resulted in violent confrontations between opponents and police, an Italian governmental commission was set up in 2006 to study all the issues. The work of the commission between 2007 and 2009 was summarized into seven papers (Quaderni). An eighth summary paper focused on cost–benefit analysis was unveiled in June 2012.
Test drilling found some fractured and sheared coal-bearing schists close to the French portal that are poorly suited for a tunnel boring machine, and old-fashioned drilling and blasting is used for the corresponding section.
The present high-altitude line
Since 1872, the Turin–Modane railway connects Turin with Lyon via the -long high-altitude (maximum tunnel altitude ) Fréjus Rail Tunnel. This initially single-track line was doubled and electrified in the early 20th century, and the Italian side of the line was renovated between 1962 and 1984, and again between 2001 and 2011. This historical line has a low maximum allowed height, and its sharp curves force low speeds. Its very poor profile, with a maximum gradient of 30‰, requires doubling or tripling the locomotives of freight trains.