A base tunnel is a type of tunnel, mainly a railway tunnel, that is built through the base of a mountain pass. This type of tunnel typically connects two valleys at about the same altitudes.
When originally constructed, classical railway lines through mountainous terrain tried to minimize tunneling, due to technical limitations and expense, and therefore required long and steep gradients and many curves, or even spirals. Tunnels were generally short and much higher up the mountain. Such tunnels are sometimes also called culmination tunnels, especially in the presence of a base tunnel through the same mountain massif.
The base tunnels take the opposite approach, minimizing or eliminating gradients and curves with the consequence of having longer tunnels but shorter total distances to travel. This allows for higher speeds and lower energy costs.
Some of the best-known base tunnels are (with length, opening and location):
Operational Gotthard Base Tunnel (, 2016), Switzerland Lötschberg Base Tunnel (, 2007), Switzerland Koralm Tunnel (, 2025), Austria Pajares Base Tunnel (, 2023), Spain Zimmerberg Base Tunnel (, planned), Switzerland Simplon Tunnel (, 1906/1921), Switzerland Apennine Base Tunnel (, 1934), Italy Ceneri Base Tunnel (, 2020), Switzerland Furka Base Tunnel (, 1982), Switzerland
Projected Mont d'Ambin Base Tunnel (, proj. 2032), France and Italy Brenner Base Tunnel (, proj. 2032), Austria and Italy Semmering Base Tunnel (, proj. 2030), Austria Valico Tunnel (, proj. 2026), Italy Ore Mountain Base Tunnel on the Dresden‒Prague high-speed line, Germany/Czech Republic ()
References