Pittsburgh and Castle Shannon Tunnel

The Pittsburgh and Castle Shannon Tunnel, also known as the Mount Washington Coal Tunnel, was a 3 ft 4 in (1, 016 mm) narrow-gauge railway tunnel under Mt. Washington.

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Tunnel data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).

Other tunnels in United States

Delaware Aqueduct United States · 137 km Päijänne Water Tunnel Finland · 120 km Orange–Fish River Tunnel South Africa · 82.8 km Ishikawa Tunnel Japan · 78.5 km Brenner Base Tunnel Austria · 64 km Mont d'Ambin base tunnel France · 57.5 km Gotthard Base Tunnel Switzerland · 57.09 km Hong Kong–Zhuhai–Macau Bridge China · 55 km