NEW YORK — From Washington to Boston, big cities and small towns buttoned up Sunday against the onslaught of a superstorm that could endanger 50 million people in the most heavily populated corridor in the nation, with forecasters warning that the New York area could get the worst of it — an 11-foot wall of water.

‘‘The time for preparing and talking is about over,’’ Federal Emergency Management Administrator Craig Fugate said as Hurricane Sandy made its way up the Atlantic on a collision course with two other weather systems that could turn it into one of the most fearsome storms on record in the United States. ‘‘People need to be acting now.’’