Passenger transport by railways is usually ensured by passenger wagons or multiple unit trains. Railway traffic on non-electrified railway tracks is still being serviced by diesel-multiple units. There are regions where buying all new rail vehicles is too expensive or unprofitable. A modernization of existing rail vehicles is one of the most effective ways to provide modern rail vehicles with a sufficient travelling standard at the acceptable acquisition costs. Passenger comfort is considered as one of the very important factors, which passengers evaluate the quality of rail vehicles from. From the dynamics point of view, oscillations of a rail vehicle body occurring while a diesel multiple unit runs on a track significantly affect the total passenger feeling. Accelerations are the consequence of oscillations that a passenger perceives in the body. The goal of rail vehicle designers is to achieve as low oscillations as possible. The main objective of this study is an investigation of passenger comfort in an existing diesel-multiple unit train, which suspension system is modified. The modification consists in the replacement of an original steel coil spring by an air-spring suspension system. This study is completely performed in a multibody simulation software, and it represents a conceptual proposal for now. There are evaluated ride comfort indices for specific running conditions. Their numerical values have revealed the improvement of passenger comfort in the case of air spring application.