The Friday File: The 35 mile Gotthard Base Tunnel in Switzerland will be the world’s longest rail tunnel when opened in 2016. Today, the world’s longest rail tunnel is the 33.5 mile Seikan Tunnel in Japan connecting the islands of Honshu and Hokkaido. The tunnel with the longest undersea portion is the Channel Tunnel at 23.5 miles. China is considering a 76.4 mile long sea tunnel crossing the Bohai Strait.