As the Seikan Tunnel — a rail link between Honshu and Hokkaido — marked the 30th anniversary of its opening on Tuesday, better coexistence between shinkansen services and freight traffic remains a key challenge.

At 53.85 km, the world's longest undersea tunnel connects the northern tip of Honshu with Hokkaido. The tunnel, 23.3 km of which runs undersea, has been a major transport artery since its opening, as it is little affected by high winds and heavy snow.

The tunnel was the world's longest railway tunnel until the opening of the Gotthard Base Tunnel in Switzerland in December 2016.