CHICAGO (CBS) — The CTA is in no hurry to put its newest ‘L’ cars back into service, despite newly-completed repairs.

Although it’s been widely reported that the new trains will return to service in early May, CTA Chief Operations Officer Peter Ousley said the 5000 series rail cars have not been formally accepted for service, and that only a few of the 54 cars will be added at a time.

“We’re still reviewing the repairs, if that’s the question,” Ousley said. “Obviously repairs have been done. We have to go back and review the quality of the repairs and determine that those repairs are acceptable.”

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He said crews also would have to undergo retraining because the cars have been out of service so long.

Construction on the new cars in New York by manufacturer Bombardier Transportation resumed Mar. 28. But Ousley said CTA is now checking the cars for any other problems extremely carefully. Each car will undergo additional testing before being released for service.

Once the bombardier construction line is operating at full capacity, CTA expects to take delivery of between two and four cars a week. In all, there are 706 cars in the order.

CTA on Dec. 16 withdrew the first 40 cars that had been delivered because of defects in steel castings in the wheel assembly that could have resulted in derailments. They had been in service for only a month.

The new alternating current-powered cars are the first of their kind at CTA and are intended to retire the two oldest series of cars still on CTA’s roster, the 2200 series manufactured in 1969-70 and the 2400 series built in 1976-77. Ousley said it is far too early to say at what point CTA will feel secure enough with the new ‘L’ cars to begin retiring the older cars.

The cars will be reintroduced on the Pink Line, where they were operating before being withdrawn. Next to receive them will be the Green Line. CTA President Forrest Claypool said he expects the new cars to begin appearing on the heavily-traveled Red Line in 2014, following repairs to the line on the North and South Sides.