The Longest Tunnel on the Line
The Kominato Railway Okubo Tunnel (小湊鉄道大久保隧道, Ōkubo Zuidō) is a concrete railway tunnel of around 1928 in the Okubo district of Ichihara City, Chiba Prefecture, and was registered as a Registered Tangible Cultural Property on May 2, 2017. At 421 meters, the Agency for Cultural Affairs calls it the largest tunnel on the Kominato Railway.
The Kominato Railway Okubo Tunnel lies about 1.5 kilometers south of Tsukizaki Station on the third section, between Tsukizaki and Kazusa-Nakano, which opened on May 16, 1928.
An S-Curve Lined with Blocks
The agency records the tunnel as concrete, 421 meters long and 3.8 meters wide, curving gently in an S.
The arch is laid in concrete blocks 33 centimeters long, while the horseshoe-shaped portals are cast in place in formwork.
The Tsukizaki First Tunnel, built two years earlier, was cast in situ throughout; here the arch is block-built, a sign of how quickly methods changed between the line's construction phases.
Seeing It
The Kominato Railway Okubo Tunnel is on a working line and cannot be entered. In normal times you experience its S-curve from a train window. If you look at a portal, stay off the track and private land.
Since Typhoon No. 25 struck in September 2026, the line between Kazusa Ushiku and Kazusa-Nakano has been suspended until further notice, with no replacement buses (company statement of September 24, 2026), so for now it cannot be reached by train.
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Q&A
- When was the Kominato Railway Okubo Tunnel built?
- Around 1928, according to the Agency for Cultural Affairs.
- How long is the Okubo Tunnel?
- 421 meters long and 3.8 meters wide, the largest tunnel on the line.
- How was the Kominato Railway Okubo Tunnel built?
- Its arch is laid in 33-centimeter concrete blocks, and its horseshoe portals are cast in place.
- Can I walk through the Okubo Tunnel?
- No. It is on a working line; ride through it on a train instead.
- Can I travel through the tunnel now?
- Not at present. The line beyond Kazusa Ushiku has been suspended since typhoon damage in September 2026.
Basic Information
| Name | Kominato Railway Okubo Tunnel (小湊鉄道大久保隧道) |
|---|---|
| Location | 802-4 Shironokoshi to 247-3 Hotoketani, Okubo, Ichihara City, Chiba Prefecture |
| Designation | Registered Tangible Cultural Property (Structure) |
| Year Designated | Registered 2017/5/2 (Heisei 29) |
| Quantity | 1 structure |
| Dimensions | Concrete, 421 meters long, 3.8 meters wide |
| Owner | Kominato Railway Co., Ltd. |
| Public Access | Working line; no access (section suspended after 2026 typhoon damage) |
References
- Agency for Cultural Affairs, Database of National Cultural Properties: 小湊鉄道大久保隧道
- https://kunishitei.bunka.go.jp/heritage/detail/101/00011476
- Cultural Heritage Online: 小湊鉄道大久保隧道
- https://online.bunka.go.jp/heritages/detail/274762
- Kominato Railway: 22 facilities registered as Registered Tangible Cultural Properties (Japanese)
- https://www.kominato.co.jp/corporate/topics/heritage/index.html
- Kominato Railway: Damage from Typhoon No. 25 and recovery outlook, September 24, 2026 (Japanese)
- https://www.kominato.co.jp/wp-content/uploads/2026/09/30216b45daba750194c363de3f54800f.pdf
Last verified: 2026.10.01
