SEPTA said Saturday that it had removed 120 Silverliner V passenger cars - nearly a third of its passenger railcar fleet - from service due to a "significant structural defect," and that the cars would remain sidelined until repairs are made.

The decision raised the prospect of considerable commuter delays when work schedules return to normal after the July 4 holiday weekend.

SEPTA spokeswoman Carla Showell-Lee said that the defects had been discovered Friday and that the decision to pull them from service was made in a conference call of senior SEPTA managers. She said that the agency still was trying to determine what the problem was, but that the cars were pulled after SEPTA staff observed that one or more of them were leaning off-center.

Bernard Norwood, head of the SEPTA conductors union, said he was told after a meeting of SEPTA officials Saturday that the defects likely did not pose a safety threat.

"They said it wasn't a serious problem but what they were doing is being very cautious," Norwood said.

It wasn't the first time that problems with the cars surfaced. Delivery of the cars, which started in 2010, was delayed because of workmanship defects and other problems; the cars also have experienced trouble with doors opening and closing during exceedingly cold weather.

In its announcement, SEPTA said that it did not anticipate that pulling the cars would impact its light service schedule through the July Fourth weekend, but the agency left open the possibility of disruptions when the normal commuter week resumes.

"SEPTA has identified a significant structural defect with its Silverliner V Regional Rail cars," the SEPTA statement said. "All 120 Silverliner V cars, which comprise approximately one-third of the Regional Rail fleet, are currently out of service for inspection. This will not adversely impact service through Monday, July 4th, as the weekend/holiday schedules can be accommodated with the remainder of the fleet."

On any given day, some 15 percent of SEPTA's 400 passenger railcars are off line for maintenance, upgrades, and other forms of service.

Pulling the relatively new Silverliner V passenger cars from service would essentially double the number of cars unavailable to carry passengers on its already crowded lines.

SEPTA said, however, that it would outline the service impacts, and detail its reasons for pulling the cars, at a rare Sunday news conference. By then, Showell-Lee said, the agency hoped to have a better handle on the problem.

"What we are putting together now is a schedule and a plan of action," she said.

SEPTA began bringing the Silverliner V cars on line in 2010, as part of a $330 million capital expenditure to meet rising demand and improve service.

The 120 Silverliner V cars were made in South Korea by Hyundai Rotem, a subsidiary of the automaker Hyundai Motor Group, and assembled in Philadelphia.

During the assembly of the new cars at a South Philadelphia factory, there were repeated issues with the malfunctioning doors, which have caused problems during intensely cold weather when they have not opened and closed reliably, delaying trains throughout the system.

The last of the cars were delivered in 2013, two years behind schedule. Work had been delayed by material shortages, design flaws, production problems, labor conflict, and workmanship errors.

Hyundai Rotem entered the U.S. market a little more than a decade ago, aggressively underbidding competitors. Its manufacturing record produced complaints, not only in Philadelphia, but by Boston mass-transit officials who had ordered cars assembled in South Philadelphia and complained of delays and shoddy workmanship.

Even so, new rail systems are typically vulnerable to operational problems. Such was the case with Amtrak's vaunted Acela, which initially was plagued with design flaws, causing many trains to be taken out of service as the rail line and manufacturer worked to make fixes.