Jammed platforms. Brutal delays. Passengers left stranded as packed-to-capacity trains sped past.

And, because some people were likely off Tuesday for the holiday, things will probably get worse, especially for riders from inner-ring suburbs who will have to contend with the most crowded trains.

"It could be a little worse tomorrow," said Jeff Knueppel, SEPTA's general manager, during a news conference Tuesday. "I wish I didn't feel that way but I've got to say it."

And the problems could last all summer, officials said, because they don't know whether a temporary weld can get the trains back in service until new beams are installed.

On Friday, SEPTA officials said they discovered cracks in a key load-bearing component of the Silverliner V's, their newest rail car, that forced the authority to pull all those cars, a third of SEPTA's total rail fleet, from service.

Tuesday was the first day of commuting since the Silverliner V's were taken off the rails. SEPTA normally runs 788 trains a day. Tuesday it ran 560.

Bella Smith, 42, waited an hour for her train Tuesday morning from Chestnut Hill West to work at the Hospital of the University of Pennsylvania.

"I was out there at 7," she said. "The 7:19 didn't show up. The 7:38 didn't come until 8."

Stations closer to the city got the worst of it, and SEPTA reiterated that people using stops such as Wayne Junction should find other means of travel. Six trains Tuesday became so overcrowded that they sped past stops on their routes without stopping.

"It was much more crowded than usual when I got on," said Cliff Sachs, 56, who waited an hour before his train arrived to take him from Trenton to his job at University City Science Center. "As we went down the line, it got to a point where it was standing room only in the aisle."

SEPTA is offering refunds to weekly and monthly pass holders, the authority said Tuesday, and is seeking additional rail cars from other transit agencies, though there likely won't be many available.

SEPTA engineers are also determining why 115 of their newest cars, some of them in service for no more than three years, are dangerously defective. They may have some answers by the end of the week, Knueppel said.

On Friday a SEPTA inspector found a 10-inch-long crack in an equalizer beam, which distributes the weight of the 146,000-pound car to the axles, near a welded part in the steel. By the end of the day, SEPTA staff found similar, though less severe, fatigue cracks on all but five of the 120 Silverliner V's.

"They don't happen quickly," Knueppel said. "It doesn't happen in days. It doesn't really happen in weeks."

Something so widespread could have only a few causes. The metal used could be the problem, the welding could have been done poorly, or the design itself could be flawed. Hyundai Rotem, a South Korean company, was awarded the $274 million contract for the rail cars in 2006 and assembled them at a plant in South Philadelphia. Workers there were quick to maintain that their craftsmanship was not to blame.

The problem, said Joe Coccio, secretary treasurer of Local 234 of the Transportation Workers Union, was "due to a defective part and has nothing to do with the manufacture of those cars in Philadelphia."

SEPTA officials said the equalizer beams were assembled outside Pennsylvania. They said they did not know the source of the steel that the beam was forged from or what company had handled the welding.

The same model rail car is used by Denver's Regional Transportation District. Those cars are newer, received in 2015, and, while that agency conducted inspections of their vehicles after learning of Philadelphia's troubles, it has not found similar fatigue cracks.

Boston's MBTA also uses Hyundai Rotem vehicles and is inspecting its cars, as well, though they are not of the same design as Philadelphia's.

"Hyundai Rotem is working very closely with SEPTA to resolve the current issues literally around the clock to get the cars back into service as soon as possible, safely and soundly," Andrew Hyer, a company spokesman, wrote in an emailed statement.

Hyundai Rotem employs 77 people at its Philadelphia facility and is bidding to build 45 bi-level cars for a $190 million contract likely to be awarded this fall.

City Hall has pushed for SEPTA to favor local vendors, including in a letter sent May 18 to Pat Deon, chairman of SEPTA's board. But the authority declined to include a local preference clause in the recent request for bidders.

Workers at Hyundai Rotem are concerned that the equalizer beam problems will reduce the company's chances of getting the contract, and Coccio suggested that SEPTA had timed the discovery of the flaw and suspension of the Silverliner V's to coincide with the Democratic National Convention at the end of this month.

"The timing of this particular announcement, we believe, has to do with SEPTA management's desire to send this work to a low bidder in China," Coccio said in a statement Tuesday.

Knueppel said again Tuesday that he and SEPTA's upper management had no warning of cracks in the equalizer beams before Friday. He and officials from the Federal Railroad Administration also said that federal law required them to withdraw rail cars with that kind of structural weakness.

SEPTA's Silverliner V's marked Hyundai's entry into the U.S. heavy rail market, and it has been a rocky beginning.

In the South Philadelphia factory where the Korean-made cars were finished, local workers fell far behind schedule as they clashed with Korean managers.

When cars did get on the tracks, communication systems didn't work right, the computer software was glitchy, and wheels slipped backward. Workers also made mistakes, such as damaging wires and drilling through wiring harnesses, SEPTA told the Inquirer in 2011.

As the new trains went into service, the agency's inspectors identified problems on the new trains and sent them back for fixes.

Factory workers said they were sometimes told to substitute wrong parts for missing parts, that they could not get answers to questions from their bosses because of the language barrier, and that the need to go back and fix problems delayed work.

Transportation advocates at the time worried that such errors could cause problems that would not show up for years. Earlier, critics also said that Hyundai Rotem, the lowest bidder for the contract, had no experience doing such work.

The Seoul-based company's problems are not limited to Philadelphia. In 2015, 40 Hyundai Rotem commuter rail locomotives were taken out of service in Boston after they were discovered to have faulty bearings, The Boston Globe reported. In that city, the manufacturer also delivered cars years behind schedule and with faulty parts, causing issues and clashes similar to those in Philadelphia during the Silverliner V rollout.

In Los Angeles, other Hyundai Rotem trains were found to have flawed parts after a deadly derailment in 2015, though an investigation had not concluded what caused the crash.

SEPTA officials said the equalizer beam cracks could have eventually caused a serious accident. Amid the inconvenience for riders, they have emphasized that everything they've done is driven by safety concerns. Some got the message.

"Safety is paramount," said Mike Bottino, 35, of Holland, "versus people being 20 minutes late."

Staff writers Daniel Block, Lauren Feiner and Steven Bohnel contributed to this article.