Nowadays, metro system is widely used for public transportation. Its regular operation consumes large amounts of electrical energy in comparison to other urban systems, while a considerable part of its non-traction energy is consumed for air exchange and ventilation of tunnels and stations. In this research, the train movement inside the four stations and connecting tunnels of underground subway system is simulated. The tunnel and station models contain important units such as ticket hall, staircases, platforms and ventilation systems.  The numerical model is validated by comparing the results with the experimental data available in the literature.  The flow field inside the tunnel and stations induced by the train movement is calculated and compared in fan-off and fan-on conditions. The results show that the train movement changes the flow direction around the fans and grille openings and can severely affect the air-exchange performance. The flow field inside the tunnels and stations is completely dependent on the piston effect caused by the train movement.  Because of the train movement, the volume OF flow exchange through station entrances, and also through station and tunnel inlets becomes ten times of that on the steady state condition with the stationary train. Also the air flow induced by the train movement is much higher than the flow generated by the air-exchange system. Therefore, the optimal use of the piston effect has a significant effect on reducing the energy consumption.