PHILADELPHIA — Commuters here faced delays and crowded trains Tuesday after a third of regional rail cars were abruptly pulled out of service over the weekend when a major defect was discovered in part of the fleet.

Officials at the Southeastern Pennsylvania Transportation Authority said inspectors had found cracks in the suspension systems of most of the commuter railroad’s relatively new Silverliner V cars. The repairs could take weeks — a signal that the misery may not end before the Democratic National Convention here later this month.

The suspension systems were still under warranty, and Hyundai Rotem, the rail car manufacturer, is working to address the problems, the transit agency said. Officials at Hyundai Rotem could not be reached for comment.

The Philadelphia region is the latest in the Northeast to face major transit disruptions over safety problems. In Washington, subway riders are grappling with closings for urgent maintenance work, while New Yorkers are bracing for the closing of a subway tunnel to repair damage from Hurricane Sandy.