Shin-Tanna Tunnel (Bullet Train Project Wartime Remnant)

The Shin-Tanna Tunnel (新丹那トンネル) is a 7,959-meter railway tunnel on the Tokaido Shinkansen, running parallel to and approximately 50 meters north of the older Tanna Tunnel (completed 1934) through the Tanna Mountains between Atami and Kannami in Shizuoka Prefecture.

The Bullet Train Project (弾丸列車計画)

In 1939, amid growing strain on the Tokaido Main Line's capacity due to military mobilization, the Japanese Ministry of Railways (鉄道省) formally adopted the Dangan Ressha Keikaku (弾丸列車計画, "Bullet Train Plan") — a project to build a standard-gauge, high-speed railway from Tokyo to Shimonoseki (Yamaguchi Prefecture) capable of speeds up to 200 km/h. The plan envisioned completing the roughly 984 km journey in approximately 9 hours, with an eventual extension through an undersea tunnel to the Korean Peninsula and onward to the Asian continent, serving Japan's imperial ambitions.

The project received formal Cabinet approval and funding in 1940. Land acquisition began along the planned route, and in 1941, construction commenced on the most challenging engineering sections — the long tunnels.

Wartime Construction (1941–1943)

Three major tunnels were started simultaneously in 1941:

1. Shin-Tanna Tunnel (新丹那トンネル, 7,959 m) — between Atami and Kannami, Shizuoka Prefecture. Work began in August 1941 from both the Atami (east) and Kannami (west) portals. By the time construction was halted, the east portal had advanced 647 m and the west portal 1,433 m, with approximately 200–300 m of concrete lining completed at each end.

2. Nihonzaka Tunnel (日本坂トンネル, 2,174 m) — between Shizuoka and Yaizu, Shizuoka Prefecture. Started in 1941, this tunnel was actually completed in 1944 and temporarily converted for use by the conventional Tokaido Main Line before being reclaimed for the Shinkansen in 1964.

3. Higashiyama Tunnel (東山トンネル, ~1,900 m) — in Kyoto. Started in July 1941, completed in August 1944 for conventional use. The tunnel's cross-section visibly changes partway through: the first 140 m from the Yamashina side was built to the smaller conventional-line gauge, after which the bore was enlarged to bullet train specifications.

In 1943 (Showa 18), with the Pacific War turning decisively against Japan, the Ministry of Railways ordered all bullet train construction halted. Resources were redirected to the war effort.

The "Shinkansen" Place Name in Kannami

During construction, worker dormitories and engineering offices were established near the western (Kannami) portal of the Shin-Tanna Tunnel. The settlement that grew up around these facilities came to be known as the "Shinkansen district" (新幹線区); according to a local history it was recognized under that name by Kannami town in 1954, a decade before the railway opened. To this day, the area retains this name, with a "Shinkansen Community Hall" (新幹線公民館) and bus stops named "Kansen-ue" and "Kansen-shita" serving as tangible reminders of the wartime project.

Postwar Revival and the Tokaido Shinkansen

The bullet train project lay dormant for over a decade after the war. In the mid-1950s, as Japan's economy recovered and the Tokaido corridor's passenger demand surged, Japanese National Railways (JNR) president Shinji Sogō championed the construction of a new high-speed line along much of the original bullet train route. The Tokaido Shinkansen project was formally approved in 1958.

Crucially, JNR was able to reuse the wartime infrastructure: the land that had been acquired, the partially excavated tunnel bores, and the completed tunnels at Nihonzaka and Higashiyama. Work on the Shin-Tanna Tunnel resumed in 1959 and was completed in 1964. The Tokaido Shinkansen opened on 1 October 1964, nine days before the Tokyo Olympics, with the Shin-Tanna Tunnel as its longest and most iconic tunnel.

The wartime bullet train project is thus recognized as the direct ancestor of the Shinkansen. The tunnels started in 1941–1944 remain in daily use, carrying Shinkansen traffic through infrastructure whose origins lie in wartime Japan's imperial railway ambitions.