Cornelius Vanderbilt built NYC’s first subway in 1841 in order to bring steam locomotives carrying Long Island Railroad passengers from Brooklyn into Manhattan wile bypassing the traffic-filled Court Street and Atlantic Avenue intersection underground. As the Verge notes: “The Atlantic Avenue Tunnel holds the Guinness world record for “oldest subway tunnel,” predating the Tremont Street subway in Boston from 1897, the 312-foot Beach Pneumatic Transit tunnel in Manhattan from 1869, and the first subway in the London Underground, which was built in 1863.”
Walt Whitman memorialized the tunnel’s closure in 1861 (after Brooklyn banned steam locomotives within its city limits): “The old tunnel, that used to lie there under ground, a passage of Acheron-like solemnity and darkness, now all closed and filled up, and soon to be utterly forgotten, with all its reminiscences.”
Reputedly, the 1611-foot-long tunnel was reopened decades later for growing mushrooms, and used during Prohibition for boot-legging, but it had remained closed and forgotten for many, many years when Bob Diamond, a local amateur archaeologist, persuaded the Brooklyn Union Gas Company to open one of its manholes and allow him to explore.
Diamond broke through a concrete wall and constructed his own staircase giving access to the tunnel, and operated his own small-scale business for 30 years, taking people (through the original man-hole) on tours of the historic tunnel.
At the end of 2010, however, jealousy of somebody else making a dollar out of an asset over which they could claim control provoked the city authorities to shut down Diamond’s tours, closing off access to the historic tunnel permanently, on the basis of safety concerns. (No one had been injured in the course of 30 years of Diamond’s operations.)
There is a bit more at Atlas Obscura.
Hat tip to Fred Lapides.