PHILADELPHIA — The Philadelphia area’s main transit agency began returning some of its sidelined rail cars to service Thursday following repairs and projects its full fleet will be back in service by mid-November.

The regional Southeastern Pennsylvania Transportation Authority pulled more than 100 coaches from service just before the Fourth of July weekend after defects were found in the suspension systems of its Silverliner V fleet.

Four repaired Silverliner V cars departed from the Fox Chase neighborhood station Thursday morning to pick up passengers, agency officials said.

Inspectors had found a fractured beam on one car and fatigue cracks on almost all other Silverliner V cars. The beams are being replaced.

SEPTA has followed “a very aggressive return-to-service schedule” and expects a return to a regular weekday schedule in early October, General Manager Jeff Knueppel said. Each car takes about 12 days to repair, and transit officials expect the new beams to last at least 30 years.

“For the most part, things are moving along very, very well,” he said.

SEPTA and South Korean manufacturer Hyundai Rotem chose a repair beam design that will not include a welded piece. Transit officials have attributed the fatigue cracks to a combination of design and manufacturing missteps, Hyundai Rotem spokesman Andy Hyer has said.

The flaw has costed SEPTA.

The agency reports $2.5 million in lost revenue in July, and it has incurred additional costs in leased equipment from Maryland and New Jersey transit services and Amtrak. SEPTA also plans to launch an express bus service after Labor Day as riders return from summer vacations and head back to school. The express service will cost the agency $100,000 a week.

The rail system linking Philadelphia and its suburbs has seen between a 10 and 20 percent drop in ridership compared to last year. Riders experienced delayed and packed trains when the faulty cars were removed from service.

“This has been a time of considerable inconvenience for our riders,” Knueppel said.