Which is the world's longest mainline railway tunnel?

The Gotthard Base Tunnel is a railway tunnel through the Alps in Switzerland, which opened on 1 June 2016 with full service to begin in December 2016. With a route length of 57.09 km (35.5 mi) and a total of 151.84 km (94.3 mi) of tunnels, shafts and passages, it is the world's longest and deepest traffic tunnel and the first flat low-level route through the Alps.

The project consists of two single-track tunnels connecting Erstfeld (Uri) with Bodio (Ticino) and passing below Sedrun (Graubünden). It is part of the AlpTransit project, also known as the New Railway Link through the Alps, which includes the Lötschberg Base Tunnel between the cantons of Bern and Valais and the Ceneri Base Tunnel (under construction, scheduled to open late 2020) to the south. It bypasses the Gotthardbahn, a winding mountain route opened in 1882 across the Saint-Gotthard Massif, which is now operating at capacity, and establishes a direct route usable by high-speed rail and heavy freight trains. It is the third tunnel connecting the cantons of Uri and Ticino after the Gotthard Tunnel and the Gotthard Road Tunnel.