The core of the project is its 70 kilometres (43 mi) long international section, which will cross the Alps through the Mont d'Ambin Base Tunnel between the Susa Valley in Piedmont and Maurienne in Savoie.

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 as well as local air pollution and of providing faster passenger transport to reduce air traffic.

After violent confrontations between opponents and police during a 2005 attempt to start reconnaissance work near Susa, an Italian governmental commission was set up in 2006 to study all the issues.

Its high elevation (1,338 m (4,390 ft)), sharp curves, steep gradients, and low tunnel ceilings make this section the bottleneck on the overall capacity of the line.

[24] Additional traffic limitations stem from the impact of excessive train transit on the population living near the line.

A 2018 analysis, by contrast, found the existing line close to saturation, largely because safety regulations now prohibit passenger and freight trains from crossing in a double-track single-tube tunnel.

[27] The path of the historical line through the deep Maurienne valley is also exposed to rockfalls, and a major landslide in August 2023 forced its closure for at least a year and a half.

[citation needed] Rather than as a natural consequence of faster transit times and a lower price for freight shipping (due to reduced energy use thanks to a much flatter profile, but without necessarily taking into account the full construction cost of the new line), they propose to increase rail traffic by coupling additional renovation of the existing rail infrastructure with sufficiently high financial incentives for rail transport and/or sufficiently heavy tolls and taxes on road transport.

This is primarily because updated safety regulations on train crossings in single-tube tunnels have significantly reduced its maximum allowed capacity.

[27] The construction of a brand-new line will also allow higher safety standards, and it will make the older infrastructure fully available for regional and suburban services, which is an important consideration near the congested Turin node.

The French section of the new line is planned with eventually separate paths for passengers and freight between Lyon and the Maurienne valley.

[36][35] In December 2024, the French government selected for detailed studies the fastest and highest capacity option, which also is the most expensive, with a mixed-use double-track line from Grenay near Lyon to Avressieux in Savoie and a single-track freight line from Avressieux to Saint Jean de Maurienne.

An underground service and rescue train station is planned around the half-way point of the tunnel, East of Modane.

While presented as a reconnaissance gallery because the project had yet to be fully approved, it was dug along the axis of the South tube of the tunnel and at its final diameter.

[15] In late 2016, that tunnel encountered a geologically difficult zone of fractured and water-soaked coal-bearing schists, and for several months made only very slow progress through it.

[39] Tunneling eventually passed this zone in Spring 2017 after injecting 30 tons of reinforcing resin,[40] and resumed at nominal speed.

[41] That gallery makes up the first 9 km (5.6 mi) of the South tube of the tunnel,[16] and it was completed in September 2019, in time and within budget.

[16][42] The contracting for the bulk of the tunnel construction was then delayed by deep disagreements on the merits of the Turin–Lyon project within the Italian coalition government between the Five Star Movement and Lega parties, and in March 2019, Italy's Prime Minister Giuseppe Conte officially asked TELT to stop the launch of tenders for further construction work.

The No TAV movement began in 1990, with actions to inform local people affected by the plans, supported by the mayors of the valley and the Comunità Montana (mountainous community).

[64] Catholics pray at the Chiomonte construction site, while other networks organise communal dinners, discussions and flash mobs.

[11] The No TAV movement generally questions the worthiness, cost, and safety of the project, drawing arguments from studies, experts, and governmental documents from Italy, France, and Switzerland.