A flawed weld that attaches a plate to a key weight-bearing beam is at the root of SEPTA's rail-car woes, the company that built the cars acknowledged for the first time Tuesday.

"The design of how it was welded and the weld itself are in question," Andrew Hyer, marketing and business-development manager at Hyundai Rotem, said Tuesday in the company's first extended comments since cracks in the beams forced SEPTA to pull one-third of its rail cars from service.

"How we decide to weld the material may make all the difference," Hyer said.

Weld problems led to fatigue cracks in equalizer beams on 115 of SEPTA's newest Regional Rail cars. Losing those cars has created a scheduling nightmare, as SEPTA has tried to keep up with demand with far fewer rail cars than it needs.

The weld work on the Silverliner V rail cars was designed and executed by HiCorp., a Zelienople, Pa., company subcontracted by Hyundai Rotem, Hyer said.

A woman answering the phone at HiCorp. Tuesday said no one at the company would comment.

SEPTA officials have said they are still reviewing tests of the equalizer beams before saying what caused the cracks.

"We are focused on making the proper fixes so that we can restore safe and reliable service to our customers," said Andrew Busch, a spokesman for the transit agency.

The welds connect plates to equalizer beams, which transfer the weight of rail cars to the axles. The plates are intended to create more surface area where the beams rest on the cars' wheel bearings. If ignored, the cracks could have led to a serious rail accident, SEPTA officials said.

Hyer did not have details on how the weld and design were flawed, but he said it was clear that the problem lay in those steps of the beam-assembly process.

Companies increasingly outsource elements of manufacturing, but a subcontractor's error does not absolve Hyundai Rotem of responsibility, said Morris Cohen, a professor of manufacturing at the Wharton School of the University of Pennsylvania.

"They're supposed to receive the parts from the subcontractor, and they have a responsibility to ensure they conform to the specifications," Cohen said. "Once they put it in their product, they take on some responsibility that what they got was an appropriate product."

The equalizer beams were tested, but were not subjected to repeated fatigue tests that simulate the pressure of years of use. Fatigue tests might have revealed the flaws, SEPTA and Hyundai Rotem said.

Hyundai Rotem will review its design and inspection process, Hyer said, but he said that competitive pressures, along with there being a limited number of companies qualified to do heavy industrial steel work, mean any company could be susceptible to such an error.

"There's no doubt in my mind that if someone else had done the work, they might have run into the same issues we have," he said.

Hyundai Rotem has taken a public flogging since the rail car problems emerged over the Independence Day weekend, Hyer said, emphasizing that the South Korean company was working intensely to help SEPTA get the cars back into operation. Its personnel have participated in testing of the cracked beams with SEPTA and with another contractor, LTK, that did an in-depth inspection of a Silverliner V car and found no other problems.

SEPTA determined that a temporary fix for the cracks would not work and is now testing two kinds of equalizer beams, one with a differently welded plate and another made of forged steel, to determine which would make the best replacement. Adding welded plates to equalizer beams is a common practice, Hyer said, adding that he did not think that had contributed to the cracks.

The testing could take two to six weeks, Hyer said, and in the meantime Hyundai Rotem has begun ordering replacement equalizer beams for all the SEPTA cars in both variations. When the testing is over, Hyundai Rotem will have the beams SEPTA prefers to use available, Hyer said. The company will absorb the cost of manufacturing the beams SEPTA chooses not to use.

"That's a risk we're taking," he said. "We are going ahead in getting different mills in line with different materials."

An open question is how much Hyundai Rotem will reimburse SEPTA for its losses while the rail operations are chaotic. Matt Mitchell, vice president of the Delaware Valley Association of Rail Passengers, estimated that it would take up to four years before SEPTA regained all the riders it has lost in recent weeks. He also said he expected that SEPTA would have to pay some portion of the costs of the 28 rail cars and four locomotives it is leasing from other transportation agencies.

"There will be a permanent ridership loss," Mitchell said. "There will be a permanent budget hit."

215-854-4587@jasmlaughlin