Montcenis Base Tunnel: Building the 57.5-km Alps Rail Link Between Lyon and Turin

Overview

The B1M tours the Montcenis Base Tunnel, a 57.5 kilometre rail tunnel deep under the Alps connecting Lyon in France to Turin in Italy. The video explains how a base tunnel lets trains pass through the mountain with a flat route, cutting journey times and reducing truck traffic.

It also explains construction approaches from both sides of the border, including drill and blast in fractured rock and seven tunnel boring machines that will lay hundreds of thousands of concrete lining segments, plus vertical ventilation shafts and safety bypasses. The piece also discusses funding, timelines, and the environmental and social challenges involved in this megaproject.

Introduction and context

The Montcenis Base Tunnel is a monumental rail megaproject spanning 57.5 kilometres under the Alps between France and Italy. The B1M explains that once complete, the base tunnel will become the longest single rail tunnel in the world, allowing high speed trains to pass through the mountain rather than around it. This shift promises to shorten travel times between Lyon and Turin and to rebalance road and rail traffic across the region, with a significant drop in truck movements and emissions.

Base tunnel concept and route

Base tunnels burrow through the base of a mountain to keep tracks flat, enabling faster, more reliable services. The Montcenis route crosses the border, with a cross-border section of about 162 kilometres comprising galleries and tunnels, and two parallel tubes for bidirectional traffic. About 45 kilometres of the base tunnel are on French soil and 12.5 kilometres are in Italy. The project also includes numerous access shafts and safety bypasses to ensure connectivity and safety along the long route.

Construction methods: drill and blast vs TBMs

Early sections of the project use traditional drill and blast in areas with fractured rock, while the majority of excavation relies on seven tunnel boring machines, including the first machine named Viviana. The TBMs are enormous and capable of cutting through hard rock, with a cutting head more than 10 metres in diameter. They install hundreds of thousands of concrete lining segments in a ring, creating a stable tunnel lining as they advance. The scale of the operation is staggering, with segments produced on-site and assembled to form continuous rings along the tunnel.

TBM operations and segment lining

Inside the TBM control cabins, operators guide the machine using multiple monitors, and jacked supports push against the lining as the cutter head advances. Each ring is formed from eight segments, with a target of around 100,000 segments in total. The process turns the TBM into a mobile factory, excavating and lining the tunnel at the same time, while excavated material is transported to the surface and reused in embankments and station areas.

Geology, exploration and ground support

The Alps present highly variable geology, so planners have conducted extensive exploratory work. The project traverses about 80 geological domains, from alluvial deposits at the entry to ancient gneiss deeper in the mountains. In some sections traditional methods are still used to shape the tunnel, while others rely on gripper TBMs and ground support such as bolts and shotcrete as dictated by ground conditions. Exploratory tunnels completed in 2022 provide critical data for forecasting ground behaviour as tunnelling proceeds.

Water management, safety and ventilation

Water ingress is a natural challenge in deep tunnels. The project employs pumping, treatment, and recycling systems to prevent flooding and reduce dust. Four vertical shafts rising up to 1300 metres will supply ventilation and escape routes. Safety protocols are strict, with workers counted in and out and location tags used to monitor personnel in the remote tunnel environment.

Governance, funding and timeline

The cross-border section is managed by Tunnel Europan Leon Turin, a 50/50 partnership between France and Italy. The European Union funds a portion of the project, and a 2025 agreement set out the timetable and actions to commission the base tunnel by the end of 2033. While most civil works contracts have been awarded, roughly three-quarters of tunnelling still remains, and the pace is expected to accelerate as TBMs come online and the remaining work proceeds across both countries.

Public response and environmental considerations

Local communities in the serene Alpine valley have organized protests, arguing environmental concerns and questioning the necessity of a new railway given the existing Lyon-Turin line. Proponents highlight green benefits such as reduced emissions and fewer landslides, and emphasize that rail is a more sustainable alternative to road freight. The Trans-European Transport Network and EU funding are also cited as benefits, helping knit together a more connected and sustainable Europe, despite the social frictions surrounding the project.

Conclusion and broader impact

As construction progresses, the Montcenis Base Tunnel stands as a landmark in mega infrastructure. It demonstrates how cutting-edge tunnelling methods, careful ground management, and cross-border collaboration can deliver transformative transport links. The project is framed as a long-term European investment in mobility, sustainability, and regional integration, with the potential to inspire future mountain infrastructure projects across the continent.