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New York City has thousands of buses that trek across the city — often in slow motion — on routes that stretch up to nearly 20 miles. Along the way, they make dozens of stops.

Then there is the B39. It goes all of 1.8 miles and makes just three stops.

This little-known bus travels from one side of the Williamsburg Bridge to the other before circling back, like a windup toy stuck on an endless loop between the edges of Brooklyn and Manhattan.

Round and round it goes. It crosses the East River 58 times a day, less often in bad traffic.

Only about 220 people ride the bus on a weekday, the city’s lowest daily bus ridership. It is a money loser for the Metropolitan Transportation Authority.