PHILADELPHIA — The Philadelphia area’s main transit agency has pulled its repaired rail cars from service after discovering a new problem.

Earlier this summer, the Southeastern Pennsylvania Transportation Authority sidelined 120 rail cars to fix beams in their suspension systems, and it began returning some of them to service this month.

But SEPTA officials said Monday that inspectors found a design problem over the weekend that resulted in occasional contact between the piece that connects the new beam and the existing equipment. Eighteen cars that had been either returned to service or were ready for service were brought back into the shop.

The problem was not a safety issue, but a design change is being made to preserve long-term equipment quality, SEPTA officials said. Those cars are expected to be available later in the week.

“Equipment was performing as expected, however, this occasional contact would ultimately require added maintenance, inspection and ultimately more repairs,” SEPTA spokesman Andrew Busch said.

SEPTA officials had pulled the agency’s entire fleet of Silverliner V cars, a third of its regional railroad fleet, from service just before the Fourth of July weekend after inspectors found a fractured beam on one car and fatigue cracks on others.

With the fleet reduced, riders experienced delayed and packed trains.

The transit agency and the trains’ manufacturer, South Korea’s Hyundai Rotem, have attributed the flaws to a combination of design and manufacturing missteps.

SEPTA General Manager Jeff Knueppel said in a statement that the agency and the manufacturer are conducting thorough tests and inspections of the equipment.

“SEPTA is committed to making sure the repair process is done correctly to ensure a long-lasting life,” Knueppel said.

Despite the new setback, the agency says it can get all the cars in working order by mid-November as planned.

The newly identified problem “does take away some capacity that we had been able to increase as we were bringing the trains back,” Busch said. The train schedule is expected to remain the same, but some riders may experience crowded cars.

The rail system linking Philadelphia and its suburbs has seen a 10 to 20 percent drop in ridership compared with last year.