The Market-Frankford Line's cars should last 30 to 40 years without major problems, such as those discovered this week.

These cars made it three years, tops.

Last weekend, SEPTA discovered cracks in vent boxes on 58 El cars that, in two cases, extended into the body bolster, a load-bearing component running the width of the car and the connection point for a wheel assembly. SEPTA pulled the cars out of service for repairs, leaving it almost 40 cars shy of the 144 needed to handle the El's rush-hour crowds.

Trains Tuesday were crowded, SEPTA said, and buses supplementing El service were well-used. The authority recommended that people avoid traveling between 7:30 a.m. and 8:30 a.m. to minimize crowding.

Seventeen years ago, when the cars were virtually new, SEPTA found cracks in the wheel assembly on eight cars. Both this week, and in 2000, welding appeared to be the reason.

A $285 million contract bought SEPTA 220 M4s that were delivered between 1997 and 1999. The M4 cars' rollout was a chronicle of malfunctions: cracks in the wheel assemblies; a shattered axle housing on one car; doors that wouldn't open or close; and seats warped under the weight of passengers, as was reported by the Daily News in a series of stories in 2000.

Those problems were repaired, but fixing cracks in the truck frame continues to be a regular task for SEPTA's mechanics. Joe Coccio, treasurer of the Transportation Workers Union Local 234 and a SEPTA mechanic, noted the truck frames are welded, rather than a single cast piece. That is a viable approach to wheel assemblies, experts said, but Larry Lee, a former rail worker and a member of the American Society of Mechanical Engineers, said a cast frame would be stronger.

"They'll come up with an engineering change, they'd implement it, and then a crack would appear in another area in the same truck," said Coccio. "They have basically been chasing these cracks around the truck for years."

Jeff Knueppel, SEPTA's general manager, said the latest problems don't appear to be related to the fixes to the truck frames, but did say the cars' construction may again be the cause of trouble on SEPTA's busiest line, which moves more than 187,000 riders each weekday.

Vent boxes were welded directly onto the body bolster, a part that bears an enormous amount of stress that causes it to flex as the train starts and stops. It's a big piece of carbon steel, 94-inches long, 23-inches wide, that is built to handle those pressures. However, the vent box, made of sheet metal, was not. Welding it to the body bolster subjected it to the same stress.

"It is a strange place to weld something," said Larry Lee, a former rail worker and a member of the American Society of Mechanical Engineers.

When cracks formed in the vent boxes, installed to provide air to prevent the traction motors from overheating, they put additional stress on the body bolsters. SEPTA has yet to determine why the boxes, which help keep the traction motors from overheating, were attached to such crucial components, and are working to determine how to fix it.

"I've seen situations where they decide to scrap the car," Lee said of cracks in a body bolster. "That's no small thing when getting into those structural components."

Both SEPTA officials and Lee, though, said the cracks weren't likely on the cusp of causing a major failure. Experts also said that while the welds may have been a major factor in the cracks, other elements could have played a role as well.

The cars were built by Adtranz, in Elmira Heights, N.Y., a company acquired by Bombardier, a Canadian manufacturer, in 2001. That company called SEPTA, spokeswoman Maryanne Roberts said, but a conversation has yet to occur. She didn't know whether a warranty with Adtranz would cover the costs of repairs.

The discovery of the cracks this weekend happened because SEPTA's inspection schedule worked, said Randy Clarke, vice president of technical services at the American Public Transportation Association. The El cars were undergoing a five-year overhaul that led mechanics to pull up floor panels, revealing the cracks underneath.

"They found it because they have the systems and processes in place," Clarke said. "This here is how you're supposed to find issues."