The urban mass rapid transit (UMRT) Line HN2A is the first light rail transit line in Vietnam. It is also the first time the operational safety assessment for the whole life cycle of a railway project is applied and assessed by an applicable scientific tool. While various industry standard methods have been deployed in many countries, their application is not appropriate for assessing the outdated railway infrastructure in Vietnam. This article proposes a method for generating safety risk models for train collisions using the fault tree analysis (FTA) technique. The FTA method comprehensively evaluates the fundamental error and failure probability that could potentially lead to accidents in general and train collisions in particular on Line HN2A. The study describes the procedure for establishing FTA and determining assumptions based on technical specifications and similar railway systems. Compared with the statistical failure results using data from operational tests and commissioning (2018–2021) of the metro line, the results here indicate that this is a reasonable theoretical model applicable to UMRT in Vietnam. The theoretical model will be processed to generate the first-ever scientific risk assessment system based on empirical evidence. In addition, real-time accident and operation data will continue to be collected and compared to the theoretical model to improve its accuracy. The findings of this study could serve as a starting point for risk management on current and future freight, passenger, and metro lines in Vietnam.