Switzerland's Gotthard Base Tunnel, opened in 2016, is the longest railway tunnel in the world. How many kilometres long is it?
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Answer
About 57 km
At its deepest the tunnel runs under about 2,300 metres of solid rock, where the mountain itself is warmer than 40 °C.
How to estimate it
Compare with the old Gotthard rail tunnel, which is 15 km long and sits about 1,100 m above sea level. A base tunnel stays flat at around 500 m, so it must begin where the valleys are 600 m lower. Alpine valleys climb about 3 percent, so that point lies 600 / 0.03 = 20 km farther out on each side. 15 + 20 + 20 = 55 km. The tunnel is about 57 km long.
Source: AlpTransit Gotthard; Swiss Federal Railways
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