The welded bogie frame is the critical safety part of the urban metro vehicle. This paper focuses on finding out the factors inducing the fatigue cracks initiated from the positioning block weld toe of metro bogie frame. Fracture morphology and metallographic analysis were conducted to identify the failure modes, and on-track tests about the dynamic stress at the positioning block weld toe and vibration acceleration were performed. The typical signals of dynamic stress and acceleration were analyzed from time and frequency domain. The relationship between wheel polygon, rail corrugation, running speed and dynamic stress in amplitude and frequency are investigated in details. Research results show that the micro cracks induced by welding at the weld toe of positioning block propagate to the spring sleeve under relatively high alternating dynamic stress, which is strongly influenced by the wheel polygon, rail corrugation and the train running speed.