The cause of the grief for SEPTA trains is an equalizer beam that transfers the weight of the 146,000-pound train car to its axles. Fatigue cracks in the area of a welded piece have made the vehicles, some of which have been in service only three years, unusable. How long the pain will last for SEPTA riders depends on whether the cracks can be temporarily welded until replacement beams arrive, or whether the authority must wait for new parts. Hyundai Rotem, which supplied the cars, is likely to play a big role in repairing the flaws, but almost a week has passed since the problems surfaced, and what that role will be is unclear.