The Seikan Tunnel (青函トンネル Seikan Tonneru) or 青函隧道 Seikan Zuidō, is a 53.85 km (33.46 mi) dual gauge railway tunnel in Japan, with a 23.3 km (14.5 mi) long portion under the seabed. The track level is about 100 m (330 ft) below the seabed and 240 m (790 ft) below sea level. It extends beneath the Tsugaru Strait — connecting Aomori Prefecture on the main Japanese island of Honshu with the northern island of Hokkaido — as part of the standard gauge Hokkaido Shinkansen and the narrow gauge Kaikyō Line portion of the Hokkaido Railway Company (JR Hokkaido)'s Tsugaru-Kaikyō Line. The name Seikan comes from combining the on'yomi readings of the first characters of Aomori (青森), the nearest major city on the Honshu side of the strait, and Hakodate (函館), the nearest major city on the Hokkaido side.
The Seikan Tunnel is the world's longest tunnel with an undersea segment (The Channel tunnel, while shorter, has a longer undersea segment). It is also the second deepest and the second longest main-line railway tunnel after the Gotthard Base Tunnel in Switzerland opened in 2016.
“This is the only section that Shinkansen and freight trains share in Japan,” said Daisuke Takahashi, the 43-year-old driver of the freight train who has 23 years of experience on the job. “Driving a train on this section is my secret pride.”
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At 30, undersea tunnel requires maintenance, need for speed