NEW YORK – From Washington to Boston, big cities and small towns Sunday buttoned up 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 New York could get slammed by a 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 U.S. "People need to be acting now."

Airlines canceled more than 5,000 flights and Amtrak began suspending train service across the Northeast. New York and Philadelphia moved to shut down their subways, buses and trains Sunday night and announced that schools would be closed on Monday. Boston, Washington and Baltimore also called off school. And non-essential government employees in the nation's capital were told not to report for work in the morning.