The Underground Railroad was a network of secret routes and safe houses established in the United States during the early to mid-19th century, and used by African-American slaves to escape into free states and Canada with the aid of abolitionists and allies who were sympathetic to their cause.
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Blackfriars Station Redevelopment Project London, England, United Kingdom. An extensive instrumentation system has been installed at the Blackfriars Station in London to monitor the station, rail tracks and surrounding buildings as the underground part of the old station is demolished and a new station constructed.
A tunnel is an underground passageway, dug through the surrounding soil/earth/rock and enclosed except for entrance and exit, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube construction techniques rather than traditional tunnel boring methods.