Watarase Keikoku Railway Kasamatsu Tunnel

Kasamatsu Tunnel: A Taisho-Era Railway Heritage on the Watarase Keikoku Line

Deep in the mountains where the border of Tochigi and Gunma Prefectures meets, the Kasamatsu Tunnel stands as a remarkable testament to Japan's early 20th-century railway engineering. Built in 1912 (Taisho 1), this 362-meter tunnel is the longest of all the tunnels originally constructed for the former Ashio Railway, a line built to transport copper ore from the once-mighty Ashio Copper Mine. Today, the tunnel continues to serve active rail traffic on the Watarase Keikoku Railway, carrying passengers through a landscape steeped in industrial history and natural beauty.

Registered as a National Tangible Cultural Property in November 2009, the Kasamatsu Tunnel is one of 38 railway structures along the Watarase Keikoku Line recognized for their cultural and historical significance — making it part of the first railway heritage registration in Japan to span two prefectures.

History of the Ashio Railway and the Kasamatsu Tunnel

The story of the Kasamatsu Tunnel is inseparable from the history of the Ashio Copper Mine, one of Japan's most important mining operations during the Meiji and Taisho eras. In 1877, industrialist Furukawa Ichibei acquired the declining mine and transformed it into a modern operation that at its peak produced nearly half of Japan's total copper output.

As production soared, the existing horse-drawn tramways proved insufficient for transporting the vast quantities of ore. A full-scale railway was needed, and the Ashio Railway Company was established to build a line connecting the mine to the city of Kiryu. Construction progressed rapidly: the first section opened in 1911, and by 1914 the entire line from Kiryu to Ashio Honzan was complete.

The Kasamatsu Tunnel was constructed in 1912 as part of this expansion, located on the section between Sōri Station and Haramukō Station — an area where steep mountain terrain and the winding Watarase River presented formidable engineering challenges. The tunnel was built using construction techniques that were state-of-the-art for the period, incorporating brick and stone masonry that has endured for over a century.

In 1918, recognizing the strategic importance of copper transportation, the Japanese government nationalized the Ashio Railway. The line became JNR Ashio Line and later, in 1989, was transferred to the third-sector Watarase Keikoku Railway, which operates it to this day.

Architectural and Engineering Features

The Kasamatsu Tunnel is a single-track tunnel extending 362 meters in a gentle curve through the mountainside. Its cross-section features a horseshoe shape using a single-center circular arch — a design commonly employed in Taisho-era railway tunnels for its structural efficiency and ability to distribute loads evenly.

The portal sections showcase a combination of construction techniques: the arch portion is built with four layers of brick masonry, while the side walls are constructed using cut stone (ashlar) masonry. This dual approach reflects the engineering practices of the era, where brick was favored for curved structural elements and stone for vertical load-bearing walls.

Inside the tunnel, six refuge niches are set into the walls, originally designed to allow railway workers to step aside safely when trains passed. These niches remain intact, preserving the tunnel's original configuration from over a century ago.

The Kasamatsu Tunnel is notable as the longest tunnel among those originally constructed for the Ashio Railway. Its position marks the boundary between Gunma Prefecture and Tochigi Prefecture, making it a symbolic gateway between the two regions.

Why It Was Designated a Cultural Property

The Kasamatsu Tunnel was registered as a National Tangible Cultural Property (Registered Cultural Property) on November 2, 2009, as part of a group registration that included 37 railway structures along the Watarase Keikoku Line. This mass registration was historically significant as the first time railway facilities spanning two prefectures were collectively recognized in Japan.

The tunnel's designation reflects several key values. First, it is an outstanding example of Taisho-era railway tunnel construction, preserving original brick and stone masonry techniques in active use. Second, it represents the engineering heritage of Japan's industrial modernization, specifically the infrastructure built to support the nationally important copper mining industry. Third, as part of the broader Watarase Keikoku Railway heritage complex, it contributes to an exceptionally well-preserved ensemble of early 20th-century railway infrastructure.

In 2016, the Watarase Keikoku Railway facility group was further recognized as a Japan Society of Civil Engineers Heritage of Civil Engineering, described as a collection of structures that conveys the spirit of the early Ashio Railway period and Japan's modern copper transport history.

Highlights and How to Experience the Tunnel

Unlike many cultural properties that require a special visit, the Kasamatsu Tunnel can be experienced simply by riding the Watarase Keikoku Railway. The tunnel is located between Sōri Station and Haramukō Station, and passengers on both regular trains and the popular open-air trolley trains pass through it during their journey.

The section between Sōri and Haramukō stations is widely considered the most scenic stretch of the entire Watarase Keikoku Line. White granite natural rock formations frame the landscape, and the trolley trains actually slow down or make brief stops here so passengers can fully appreciate the views. The Kasamatsu Tunnel is the third tunnel after departing Sōri Station heading toward Ashio.

For the best experience, consider riding the Trolley Watarase Keikoku-gō, a diesel locomotive-hauled train with open-air carriages that operates from April through November. The open windows allow you to feel the rush of cool mountain air as you enter the tunnel and hear the echoes of the train reverberating off century-old brickwork. During the journey through the preceding Kusaki Tunnel, LED illuminations on the ceiling create a starry sky effect — a magical prelude to the heritage tunnels that follow.

The area surrounding the tunnel is characterized by dramatic mountain scenery, with the Watarase River carving through steep gorges below. Autumn brings spectacular foliage of Japanese maple and mountain ash, while fresh green foliage in early summer creates an equally stunning landscape.

Nearby Attractions

The Kasamatsu Tunnel is situated along a corridor rich in historical and natural attractions, making it easy to combine a tunnel experience with broader exploration of the area.

Ashio Copper Mine Tourist Site (足尾銅山観光) — Located near Tsūdō Station, this facility allows visitors to ride a small trolley into an actual mine shaft and see life-size dioramas depicting the harsh conditions of copper mining through the centuries. It provides essential context for understanding why the railway and its tunnels were built.

Sōri Station (沢入駅) — The nearest station to the tunnel's south portal, Sōri Station itself is a registered cultural property with a charming stone platform and wooden waiting shelter dating from the railway's early days.

Kusaki Lake and Dam (草木湖・草木ダム) — A scenic reservoir surrounded by mountains, ideal for a peaceful walk. The area is particularly beautiful during cherry blossom and autumn foliage seasons.

Gōdo Station Restaurant "Seiryū" (列車のレストラン清流) — At Gōdo (Kōbe) Station, two retired Tōbu Railway Deluxe Romance Car carriages have been converted into a unique train restaurant where you can enjoy local cuisine in a nostalgic railway setting.

Tomihiro Art Museum (富弘美術館) — Accessible via bus from Gōdo Station, this museum showcases the inspirational flower paintings and poetry of Hoshino Tomihiro, created by mouth after he became quadriplegic.