Abstract: Intelligent railways represent a pivotal direction in Chines railway advancement and the space-air-ground integrated network will become one of the key information infrastructure for intelligent railways in the future. This paper addresses key challenges faced by the space-air-ground integrated network for intelligent railways and proposes a framework of security assurance technologies for the integrated network focusing on security perception, collaborative reasoning, and defense decision-making. Furthermore, it explores key technologies such as network security knowledge graph construction, network attack detection and path graph generation, and dynamic collaborative defense strategies, which can provide reference for the security construction of the space-air-ground integrated network for intelligent railway.