The implementation of the general objectives of environmental and climate policy implies significant involvement of low-carbon transport. High-speed rail (HSR) transport is particularly interesting, which is, unfortunately, usually not profitable for the owner of this infrastructure. Redirecting passenger and freight traffic from the road to rail improves the economic performance of the HSR, while improving the performance of the environmental protection system by reducing the amount of fossil fuel used by the transport sector and greenhouse gas emissions. The aforementioned economic and ecological benefits usually precede investments in the modernization of the existing railway infrastructure. The aim of this paper is to evaluate the share of environmental benefits in comparison with other benefits on the example of the project of reconstruction and modernization of the railway in Serbia using cost-benefit analysis. The financial analysis of the reconstruction and modernization project for the Novi Sad – Subotica – the border with Hungary railway line has not shown a satisfactory level of financial internal rate of return (FIRR), and the final decision on the implementation of the project can be made after carrying out a socio-economic analysis. The socio-economic analysis of the project pointed to the significant contribution of the project to the overall socio-economic well-being of the country and/or region. In the structure of the overall socio-economic benefits from the realization of the project, the savings due to reduced pollution of the environment is predominant, especially in freight traffic. This indicates a high ecological contribution of this and similar projects to the community.