In car after car, the trouble spot was the same:

Hairline cracks along a joint where contractors had welded steel plates no bigger than an ice cream sandwich.

On Thursday, SEPTA engineers offered the first public look at the cracks that sidelined 120 rail cars, upending the daily commute for thousands of passengers since early July.

Still unclear is what caused the cracks, in most cases barely noticeable to the untrained eye, but officials said they hope to have an answer next week.

Only then will engineers be able to decide the best course for getting the Silverliner V cars back in action, said Ron Hopkins, the transit agency's assistant general manager.

"As much as we'd like to say what that fix is, there is a lot that goes into it," he said during a tour of the agency's Overbrook maintenance facility. "We're not there yet."

Agency officials have been warning riders to expect abbreviated service and delays at least through the end of summer. They now say it may take even longer to implement a complete solution, but are reluctant to guess at a time frame before knowing what the fix involves.

The cracks originated in the area where the plates were welded. Mechanical engineers not connected with SEPTA say that suggests one of three possible causes:

An issue with the type of metal used; a problem with the way it was welded; or a design flaw - meaning perhaps the rail cars should not have been equipped with welded plates.

The plates were welded to each end of nine-foot steel beams called equalizers, which help to distribute the weight of the 146,000-pound cars to their wheels.

Each equalizer is 11/2 inches thick, cut from a sheet of steel. The welded plates on the end of each equalizer are 21/2 inches thick, allowing the weight to be distributed over a wider area.

In the past, some rail cars were designed with equalizers forged from one piece of steel, fashioned with a wider base, so there was no need to weld extra plates.

"Hindsight is always great," Hopkins said when asked if he wished the cars had been designed without welded plates.

The problem was discovered July 1 when a SEPTA employee noticed a car leaning slightly.

Inspection revealed that a crack had spread from the area of the welded plate all the way through the equalizer. That prompted a review of all Silverliner V cars, the first of which were put into service in 2010.

Greg Buzby, a senior project engineer for the transit agency, got the call to come in that day at 6:20 a.m. One by one, the cars were jacked up so he and colleagues could conduct a thorough review.

"You're at the crime scene," Buzby said. "You're checking everything."

Cracks were found in 115 of the 120 cars, always in the area where the plates were welded on.

Most were small. To make one of the cracks more visible for reporters, SEPTA employees sprayed on a penetrating red dye.

"It's important to understand, this is not a very simple thing to see," Hopkins said.

Hopkins would not speculate how the problem would be fixed, whether it involves stouter equalizers or some other design tweak.

The answers will come from tests of metallurgical properties of the equalizers and the plates welded to each end, Hopkins said.

"The data has started to come in," Hopkins said. "We need to understand it. I'm hoping next week. The more we know, the sooner, the faster, we can figure out our path."