Abstract: Traditional system architecture constructing methods focus on top-down design from the perspective of the overall complex system, lacking fine-grained research on the intelligent behavior of multi-level entities in the system and the collaborative linkage mechanisms between entities. Based on the theory of Complex Adaptive System (CAS), this paper studied intelligent railway systems to meet the requirements of high coordination, high aggregation, and high interaction among various entities in intelligent railway systems. By analyzing the CAS characteristics of intelligent railway systems, the paper proposed the composition and structure of CAS entities in intelligent railway systems, and then constructed the technical system and data system of intelligent railways, which could provide new methods and ideas for the design and optimization of intelligent railway systems.