Abstract: An intercity train gearbox cracked, resulting in oil leakage. The failure reasons were analyzed by macroscopic inspection, chemical composition analysis, metallographic inspection, scanning electron microscopy and tensile test. The results show that the failure mode of the gearbox was stress corrosion cracking. The gearbox was rigidly suspended during assembly, causing internal stress at the corner position of the closing surface, which became the source of cracks. The service environment of the gearbox was humid and contained chloride ions, and the closing surface of the gearbox was at the casting riser position, whose microstructure had deteriorated and declined, which further promoted the initiation and expansion of cracks. It was recommended to improve the casting process, optimize the gearbox structure design and assembly gluing process to prevent such failures from reocurring.