A three-dimensional flow field induced by two trains passing by each other inside a tunnel is studied based on the numerical simulation of the three-dimensional compressible Euler/Navier-Stokes equations formulated in the finite difference approximation. A domain decomposition method with the FSA (fortified solution algorithm) interface scheme is used to treat this moving-body problem. The computed results show the basic characteristics of the flow field created when two trains pass by each other. The history of the pressure distributions and the aerodynamic forces acting on the trains are the main areas discussed. The results indicate that the phenomenon is complicated due to the interaction of the flow induced by the two trains. Strong side forces occur between the two trains when the front portion of the opposite train passes by. The forces fluctuate rapidly and the maximum suction force occurs when two trains are aligned side by side. The results also indicate the effectiveness of the present numerical method calculating moving boundary problems.