How do you dig the world's longest railway tunnel through the Alps?

You do not dig it from two ends: you attack it from many places at once. To cut construction time in half, four access tunnels were built so that digging could start at four different sites at the same time: Erstfeld, Amsteg, Sedrun and Faido. At Sedrun, a 1 km access tunnel leads to two vertical shafts that go 800 metres down to the level of the base tunnel. The result is the Gotthard Base Tunnel in Switzerland, which opened in June 2016 and, at 57.09 km, became the world's longest railway tunnel (as of 2016). Four giant tunnel boring machines, nicknamed Sissi, Heidi, Gabi I and Gabi II, did much of the cutting.

It lies deep under the Alps, to a maximum depth of 2,450 metres, comparable to the deepest mines on Earth, and without ventilation the temperature inside the mountain reaches 46 degrees Celsius. The two tunnels are joined about every 325 metres by connecting galleries. It is the first flat route through the Alps, at a maximum elevation of 549 metres above sea level, the same as the city of Bern.

Why dig so far down? The tunnel is meant to shift freight from trucks to trains, which reduces environmental damage and deadly road crashes. The old route, the Gotthard railway line, was a winding mountain route opened in 1882 that was operating at its capacity. The new tunnel is called a base tunnel because it bypasses most of that line.

The human cost was real. Nine workers died during construction, and a shrine to Saint Barbara, the patron of miners, stands inside the tunnel as a memorial.