Commuters to New York City were warned to expect crowded and delayed trains Monday morning while federal authorities began an investigation into what caused a commuter train to derail early Sunday morning, killing four people and injuring 60 others. The accident, near the Spuyten Duyvil station in the Bronx on Metro-North’s Hudson Line, caused the train’s locomotive and its seven cars to topple over and left the lead car inches from the water.