Public transport tendering procedures constitute a complex decision-making process that requires taking into account many technical, economic, environmental, and social factors. Traditional cost-minimization approaches are no longer sufficient, because supplier selection decisions must balance quality, innovation and long-term profitability of the investment. Multi-criteria decision making (MCDM) is therefore emerging as a valuable tool for effectively comparing different tender offers, based on a wide range of criteria. The article presents the application of one of the MCDM methods, the weighted sum method, in the process of selecting a supplier of rail vehicles, taking into account specific technical and operational requirements, as well as the principles of sustainable development. The article aims to present the benefits of implementing multi-criteria decision support methods in tendering procedures, offering a tool that not only supports the supplier selection process but also ensures a transparent justification of the decision based on comprehensive, multi-faceted analyses. The aim of the article is to present and analyze the application of the weighted sum method as a multi-criteria decision support tool in four tender procedures conducted by Koleje Mazowieckie for the procurement of rail vehicles equipped with innovative power sources. The adopted research methodology, which involved analyzing the offer evaluation criteria, led to the conclusion that the weighted sum method enables the ordering party to adjust the weights assigned to individual criteria according to its needs and priorities.