The issue of assessing socio-economic impacts represent a key element of the decisionmaking process on the implementation of major public investment projects. The correctness of the decision depends both on the chosen principle of the socio-economic analysis and the input data. The presented article focuses on updating selected input values for the socio-economic assessment of railway infrastructure projects. Specifically, the simplified values of the estimated rail accidents costs. Accident costs are used for considering the change in the safety. At present, these values, which are also stated in the national methodological resources, are based on statistical data of the entire European Union and thus do not reflect the possible national specifics of projects implemented in the territory of individual Member States. The principle of updating values is from a methodological point of view based on the original calculation principles, however, involves a set of information items on the occurrences that emerged in the past in a specific area. The output of the article is a set of methodological steps considering national conditions when determining the average accident costs, subsequently verified on a case study of the railway network in the Czech Republic. The outputs of the presented article directly build on the results of the research project in which the team of article authors has been involved. The research results refer to different values of accident costs uniformly determined for the entire European Union territory and those determined individually for the conditions of the railway infrastructure in the Czech Republic.