This 7.8 km long tunnel shortened the trunk route between Tokyo and Kobe by omitting a detour round the mountains between Atami and Numazu.

[1] The initial routing of the Tōkaidō Main Line railway, connecting Tokyo with Osaka, avoided the Hakone mountains between Shizuoka and Kanagawa Prefectures by taking a long detour north to Gotemba and then south to Numazu.

Recognizing that the Gotemba loop created a significant bottleneck in the rail system, the Japanese Railroad Ministry issued a contract in 1918 to the Kajima Corporation to build a tunnel through the Hakone mountains.

The project was promoted as a major public works initiative intended to help boost Japan’s economy following the post-World War I economic recession.

The Hakone mountain range is an active volcanic zone, containing a number of faults and frequently experiencing earthquakes.