The secondary suspension of most new EMU and DMU rail vehicles is equipped with air springs, to offer a good ride comfort to passengers. Air springs are a very important isolating component, which guarantees good ride comfort during the trip. In most rail–vehicle models developed by researchers, the thermo–dynamical effects of air springs in the rail–vehicle dynamics are not considered and secondary suspension is modeled by simple springs and dampers. As the performance of suspension components, especially for air springs, have significant effects on rail–vehicle dynamics and the ride comfort of passengers, a complete nonlinear thermo–dynamical air spring model, which is a combination of two different models, is introduced. Results from field tests show remarkable agreement between the proposed model and experimental data. Effects of air reservoir volume and the connecting pipes' length and diameter on the system performances are investigated here.