The Longest of the Kanbai Bores
At 166 meters, the Dai-ichi Kanbai Tunnel is the longest of the brick tunnels registered along the Watarase Keikoku line, driven through a ridge at Kirihara in the Omama district of Midori, Gunma Prefecture. It carries a single track. The portals are cut as a horseshoe with brickwork four courses deep, and the lining tells a small structural story as the eye travels down the wall: the arch overhead is built entirely of brick, while the lower haunches combine brick with courses of cut stone, an ashlar facing set where the wall meets its heaviest load.
The tunnel dates to 1912 and was reworked in 1974, so what a rider passes through today carries both its original fabric and a later refit. It does not stand alone. The Dai-ni and Dai-san Kanbai tunnels lie in close sequence with it, and the three were built one after another through the same run of ridges. Together they form a continuous piece of the gorge's engineered landscape, a rhythm of dark and light that begins where the line leaves the open valley near Omama and turns into the narrow scenery upstream.
A Mine Railway's Structures, Kept as Heritage
The line began as the private Ashio Railway, opened in 1912 to bring copper ore down the Watarase gorge from the Ashio mine, one of the largest in Asia and the site of the Ashio copper poisoning, an early industrial pollution case that damaged the river valley. The railway was nationalized in 1918 as the government's Ashio Line, and after the mine closed in 1973 it became, in 1989, the third-sector Watarase Keikoku Railway that runs the route today.
The Dai-ichi Kanbai Tunnel was registered as a Registered Tangible Cultural Property on 2 November 2009, with notice on 19 November, as one of about three dozen line structures listed together. Registration is a lighter category than the "Important Cultural Property" designation, protecting structures generally more than fifty years old while keeping them in daily use. The listing recognizes the tunnel's contribution to the historic landscape of the country. Nearby, the Kami-Kanbai station building, listed a year earlier in 2008, marks the same stretch of line.
Riding the Gorge Where It Narrows
The Dai-ichi Kanbai Tunnel sits about 240 meters southeast of the Dai-ni Kanbai Tunnel, so the two arrive in quick succession as a train works upstream. This is roughly where the Watarase Keikoku line trades the towns below Omama for the wooded gorge, and the run of Kanbai tunnels marks that transition. Watching the lower walls as the carriage passes, an attentive rider can catch the change from all-brick arch to the mixed brick and cut stone below.
The tunnel is live railway and cannot be entered or walked. The way to experience it is from a train, and the open-sided torokko sightseeing carriages that run in the warmer months make the succession of bores especially immediate. For anyone tracing the line's engineering, the Kami-Kanbai station nearby and the neighboring second and third tunnels reward a slower journey up the valley.
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Q&A
- How long is the tunnel?
- It runs 166 meters, the longest of the brick tunnels registered along this line. It carries a single track.
- What is distinctive about how it is built?
- The arch is all brick, while the lower walls combine brick with courses of cut stone. The portals are horseshoe-shaped, four courses of brick deep.
- Are there other Kanbai tunnels?
- Yes. The second and third Kanbai tunnels follow in close sequence, built one after another through the same ridges. The second is about 240 meters to the northwest.
- Can I visit it?
- No. It is active railway and closed to pedestrians. You can see it from a train on the Watarase Keikoku line.
Basic Information
| Name | Watarase Keikoku Railway Dai-ichi Kanbai Tunnel |
|---|---|
| Location | Kirihara, Omama-cho, Midori City, Gunma Prefecture |
| Designation | Registered Tangible Cultural Property (registered 2 November 2009; notice 19 November 2009) |
| Registration No. | 10-0299 |
| Count | 1 structure |
| Date | 1912; refitted 1974 |
| Structure | Brick and stone construction, 166 m long, with retaining wall |
| Access | Viewable from trains on the Watarase Keikoku line; the tunnel is not open to the public |
References
- National Database of Designated Cultural Properties, Agency for Cultural Affairs — Watarase Keikoku Railway Dai-ichi Kanbai Tunnel
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00007872
- Watarase Keikoku Railway Co., Ltd. — Company History
- https://www.watetsu.com/company/history.php
- Watarase Keikoku Line (Japanese Wikipedia)
- https://ja.wikipedia.org/wiki/わたらせ渓谷鐵道わたらせ渓谷線
Last verified: 2026.07.02