The Kamonomiya model track, as engineers called it, opened in June 1962 as the critical test bed for Shinkansen technology. Over two years, prototype trains logged a remarkable 250,000 kilometers on this section alone. On October 27, 1962, a test consist designated 'set B' reached 190 km/h, breaking the previous Japanese rail speed record. Four days later, on October 31, the same set pushed past 200 km/h for the first time -- a psychological barrier that proved high-speed rail was not merely theoretical but achievable. The Bentenyama Tunnel, sitting along this stretch near the Kozu area of the Oiso hill land, was part of every one of those test runs. Every record-setting sprint passed through its 1.3 kilometers of concrete and rock.

The geology beneath Bentenyama tells its own story. The tunnel cuts through the southwestern edge of the Oiso hill land, a region characterized by diluvium-tertiary formations -- ancient layers of compacted soil dating back tens of thousands of years. Alongside the tunnel's path, the hill is composed of alternating beds of sand gravel and tuffaceous mudstone, volcanic sediment compressed over geological ages. For the engineers of the early 1960s, boring through this layered sandwich of soft and hard material was a significant challenge. The tunnel had to withstand not only the passage of trains at unprecedented speeds but also the seismic activity common to this stretch of the Japanese coast, where tectonic plates grind beneath the surface.

Japan's bullet train did not spring from nothing in 1959, when construction formally began. The idea traced back to the Dangan Ressha project of 1939, a wartime plan to build a high-speed wide-gauge railway along the Tokaido corridor. The government acquired land and began boring tunnels before the Pacific War consumed all available resources and the project was shelved. Three tunnels from that era were later incorporated into the Tokaido Shinkansen route. The Bentenyama Tunnel itself was built fresh for the new line in 1962, but it inherited the vision of those earlier planners. When the Diet approved the modern Shinkansen project in December 1958, allocating 194.8 billion yen for construction, chief engineer Hideo Shima and Japanese National Railways president Shinji Sogo were fulfilling a dream that had survived wartime defeat and postwar reconstruction.

When the Tokaido Shinkansen opened on October 1, 1964, nine days before the Tokyo Olympics, it connected Tokyo and Osaka in four hours. The Hikari express and the all-stops Kodama began their runs along a route that remains one of the busiest rail corridors on Earth. The Bentenyama Tunnel, operated today by Central Japan Railway Company, sits between Shin-Yokohama and Odawara stations along this route. A modern Nozomi train, traveling at up to 285 km/h, passes through its 1.3 kilometers in roughly 17 seconds. Passengers barely register the brief dimming of light from their windows. Yet this unassuming stretch of tunnel near the Kanagawa coast represents one of the most consequential leaps in transportation engineering of the 20th century -- the place where speed stopped being a fantasy and became a timetable.