SEPTA's Silverliner V cars were sidelined by a problem that has dogged mechanical engineers for as long as they have worked with metal: fatigue.
That is the term for microscopic cracks that can develop with repeated back-and-forth loading, which happens as trains move from one section of track to the next. If allowed to spread, such cracks can eventually lead to failure, as when you bend a paper clip enough times that it breaks.
Most of the SEPTA cars were pulled out of service before that happened, but in one car the crack had propagated through a 9-foot beam called an equalizer, agency engineers said Tuesday.
"These fatigue cracks are progressive. They don't happen quickly," said Jeff Knueppel, the agency's general manager. "It's a months, year kind of issue."
Each of the 146,000-pound cars has four equalizers, which help to distribute the weight evenly across eight wheels - two per equalizer. The cracks originated in the area where the equalizers rest on the wheel bearings, but exactly why remains unclear.
Larry Lee, a member of the American Society of Mechanical Engineers who has worked for railroads, said cracks can be initiated by various causes.
For example, surface imperfections on a piece of metal can lead to concentrated stress when loads are applied.
"There may have been some minor, even microscopic flaw in the metal," Lee said.
Another possibility is the type of metal itself. Welded to the end of each equalizer are plates where the beams rest on the wheel bearings, and added stress can arise if the metal used in the equalizers was incompatible with the metal used to make the plates or the welding wire, Lee said.
The equalizers were made from a malleable type of steel in a category of alloys called T1, SEPTA officials said. They did not have details Tuesday on the types of metals used in the plates and the welds.
Joe Martin, a Drexel University professor of civil engineering, said the discovery of cracks in so many cars - 115 out of 120 - indicates some sort of systemic problem.
That could mean a design flaw, such as if the equalizers were not stout enough to handle the job; a materials flaw, perhaps even with a subpar batch of steel; or a flaw in the construction process, Martin said.
The cars initially were designed to weigh 136,000 pounds, but 10,000 pounds of metal were added to meet federal regulations that require a certain stiffness in the car's body, SEPTA's Knueppel said.
The equalizers were not beefed up in response.
But on another railroad 80 years ago, engineers took a different tack. Air conditioning equipment added to iconic Pullman railway cars in the 1930s increased each car's weight by more than 12,000 pounds, said Lee, of the mechanical engineering society.
Thicker equalizers were installed in each car to compensate.
Lee and Martin both said the added 10,000 pounds of weight in the case of the SEPTA cars was probably not enough to warrant thicker equalizers, but they could not say for sure.
"They should do the calculations to see if it still has an adequate margin of safety," Lee said. "Or if it doesn't."