This incredible new £21.1 billion high-speed rail mega-project will connect the two European cities of Milan and Paris and dramatically reduce passenger travel times, allowing the land route to becomre more competitive with plane travel.

The Turin-Lyon high-speed railway, about 170 miles long in total, is intended to link Italian and French networks. The international section, the only part where construction has started, will cross the Alps through the Mon d’Ambin Base Tunnel between Susa Valley in Piedmont and Maurienne in Savoie.

This 37.5-mile tunnel will become the longest rail tunnel in the world, ahead of the 35.3-mile Gotthard Base Tunnel which runs through the Alps in Switzerland.

The total cost of the line has been estimated to be 25 billion euros – or £21.2 billion – with the international section alone set at eight billion euros – or £6.7 billion.

The network has several aims. Firstly, the transfer of freight traffic across the Alps from trucks to rail to reduce CO2 emissions and local air pollution. Secondly, it will provide faster passenger transport to reduce air traffic.

The new line will have a maximum gradient of 12.5 percent, compared to 30 percent for the old existing line, with a maximum altitude of 580 metres instead of the previous 1,388 metres. It will also have much wider curves. This will allow heavy freight trains to transit at 62 mph and 140 mph for passenger trains, significantly reducing travel time and also energy usage and costs.

As such, passenger travel time from Milan to Paris is set to be cut from seven hours to just four, meaning the network will become time competitive with plane travel.

However, it must be noted that despite the name often used, the line is not high-speed under the definition used by the European Commission – its design speed is 12 percent below the 155 mph threshold used by the commission to define high-speed railways. The line is therefore part of the TEN-T Trans-European conventional rail network within its “Mediterranean Corridor”.

The European Union is funding 40 percent of the tunnel costs, and has indicated willingness to increase its contribution to 55 percent, as well as to help fund its French accesses if those go beyond mere adaptations of the existing infrastructure, according to telt.eu.

A new ashlar factory – thin squared and dressed stone – in La Chapelle, France, is due to come online in June 2024, serving Lot 2 of the base tunnel. The segments produced, each weighing ten tonnes and built to line the tunnels, are the result of an innovative process relying on extreme automation and maximum attention to sustainability.

While it takes ten minutes to produce a segment in a standard factory, the new factory can produce them in just five and a half minutes. This ensures the production of 160 items a day, according to webuildgroup.com.

The project has been criticised for many reasons. In the French town of Chimilin, which will be split in two by the railway, the town council has opposed the plans since 1992 and the previous mayor said that the economic uncertainty has been damaging for the region. The Italian No TAV movement began in the Susa Valley in 1990, and questioned the project’s worthiness, cost and safety.

Other criticisms focus on the cost, as a result of the fact that traffic in the area was decreasing when the project was decided, and the argument that plane travel is still faster than the full route, including time to and from the airport and through security.

Civil engineering work first started in 2002, with the construction of access points and geological reconnaissance tunnelling. In 2016, a 5.6-mile gallery was tunnelled from Saint-Martin-de-la-Porte towards Italy and was completed in Sept 2019, according to Le Dauphine. In late 2022, the expected completion date for the base tunnel was set for 2032.