The Seikan Tunnel (青函トンネル Seikan Tonneru) is the longest railway tunnel in the world, at 53.9 km (33.4 miles). It connects the islands of Honshu and Hokkaido in Japan as part of the JR Kaikyo Line.
The double-track railway inside the tunnel is built with three rails, so that it can handle both narrow gauge and standard gauge trains. Currently, only narrow gauge trains use the tunnel: there are long-term plans to link the tunnel into the Shinkansen network, although these are not expected to be completed until after 2020.
There are two train stations inside the tunnel (Yoshioka-Kaitei Station and Tappi-Kaitei Station), both of which are museums detailing the history and function of the tunnel.