The Mianus River Railroad Bridge, also known as the Cos Cob Bridge, is a bascule bridge built in 1904 over the Mianus River, in Greenwich, Connecticut. It was listed on the National Register of Historic Places in 1987. The bridge carries the Northeast Corridor, the busiest rail line in the United States, both in terms of ridership and service frequency. It is operated by the Metro-North Railroad, successor to Conrail, Penn Central, and the New York, New Haven and Hartford Railroad, which erected it, and is owned by the Connecticut Department of Transportation.
It is a rolling lift type moveable bridge, and was prefabricated by the American Bridge Company, to replace a previous bridge on the site which was unsafe.
The nearby Cos Cob Railroad Station is also NRHP-listed, as well as the Cos Cob Power Station, a former New Haven Railroad electrical substation on the western edge of the bridge that despite being demolished during the turn of the millennium, is still listed on NRHP. It is one of eight moveable bridges on the Northeast Corridor through Connecticut surveyed in one multiple property study in 1986. The eight bridges from west to east are: this Mianus River Railroad Bridge, at Cos Cob, built in 1904; Norwalk River Railroad Bridge at South Norwalk, 1896; Saugatuck River Railroad Bridge at Westport, 1905; Pequonnock River Railroad Bridge at Bridgeport, 1902; Housatonic River Railroad Bridge, at Devon, 1905; Connecticut River Railroad Bridge, Old Saybrook-Old Lyme, 1907; Niantic River Bridge, East Lyme-Waterford, 1907; and Thames River Bridge (Amtrak), Groton, built in 1919. The Pequonnock River bridge—also on Metro-North's New Haven Line, as are the Norwalk, Westport, and Devon bridges—has since been replaced.