In order to achieve the goals of the EU transport policy it is necessary to improve the quality of passenger transport rail services. One of main reasons for the poor quality of these services is the outdated rail vehicle fleet. European Structural and Investment (ESI) Funds provide a unique opportunity for the modernisation of this fleet. However, the issue arises whether the public funding of rolling stock investments constitutes State aid within the meaning of Article 107(1) of the Treaty on the Functioning of the European Union (TFEU or Treaty). In this case, could the aid be justified, i.e. could such aid be declared compatible with the EU internal market? The article presents the Polish experience in this field.