Abstract: The subway train bogie frame was mainly welded by St52-3 steel plate. The bogie frame fractured after the subway train run for about 5 years. The macro/micro structure, chemical composition, tensile and fatigue properties were analyzed by optical microscopy, scanning electron microscopy, materials testing and so on. The results show that fatigue cracking occurred on the bogie frame which initiated in the welding defects of solder connection part. The subway train never occured fatigue cracking during nearly 10 years′ operation after improving welding quality and changing the stress structure of bogie frame.