SEPTA removed its Superliner V railcars from service after structural defects were discovered:
SEPTA has identified a defect with its Silverliner V Regional Rail cars that has resulted in these trains being taken out of service for the immediate future. This will impact customers starting Tuesday, July 5, as SEPTA’s passenger capacity for weekday travel will be reduced. All 120 Silverliner Vs, which SEPTA received between 2010 and 2013 and comprise approximately one- third of the Regional Rail fleet, are out of service.
The Silverliner V structural defect was discovered early Friday morning by SEPTA railroad vehicle maintenance personnel. Follow-up inspections with the fleet showed that there was a problem with cracking in the main suspension systems. Within 24 hours, all Silverliner Vs had been taken out of service. SEPTA will work with Hyundai Rotem, the rail car manufacturer, to resolve the problems. The suspension systems are still under warranty, and Hyundai Rotem is working cooperatively with SEPTA to locate and expedite the procurement of materials to repair or replace the failed suspension components.
These Hyundai-Rotem railcars have been an ongoing headache for SEPTA (and other agencies):
It wasn’t the first time that problems with the cars surfaced. Delivery of the cars, which started in 2010, was delayed because of workmanship defects and other problems; the cars also have experienced trouble with doors opening and closing during exceedingly cold weather.
Hyundai Rotem entered the U.S. market a little more than a decade ago, aggressively underbidding competitors. Its manufacturing record produced complaints, not only in Philadelphia, but by Boston mass-transit officials who had ordered cars assembled in South Philadelphia and complained of delays and shoddy workmanship.
[…and also Metrolink in So. California]
There is a large worldwide market for commuter trains. They come with competitive prices and reliable service histories. But instead of using any of those proven designs, SEPTA wanted trains built locally, and designed to an obsolete government spec. And so while it is easy to blame Hyundai, the real culprit is Buy-America policies.
Systemic Failure 
Mitchell noted that the trucks were not manufactured by Hyundai Rotem but by a third party.
Trucks had to come from US supplier (Columbus Castings who are now in Bankruptcy) to meet Buy America requirements.
http://marketurbanism.com/2016/03/11/protectionism-is-already-harming-american-workers-and-cities/
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Of course, it’s the feds who forced SEPTA into the Buy America nonsense. From what I know of SEPTA, they’d happily buy anything off the shelf.
The more interesting question in my view is how it is that a heretofore reliable truck design produced for decades becomes a second rate, defective product. when applied in this case.