In this paper, we address the railway safety assurance issues, impacting the conventional railway safety process due to the introduction of autonomy and AI technologies. Concretely, based on the system hierarchy and the railway safety lifecycle, we elaborate a methodology for the safety assurance of autonomous train and its operations. The proposed methodology consists of a set of safety processes and activities to be conducted, in parallel to the conventional ones, at three hierarchical system levels ; namely: (i) the overall system level (i.e., train); (ii) the AI-based component level (i.e., perception and decision-making components); (iii) and the AI software level (i.e., inference and decision learned algorithm).