The phenomena related to the transit of high speed trains in tunnels was investigated using a 2D numerical model. It showed the stationary pressure field associated with the train and the superimposed shock wave generated when the train enters the tunnel. This wave travels at the sound speed forwards and backwards along the tunnel, partially reflected back and partially radiated outdoors.
This sequence of images shows the pressure waves generated when a train enters a tunnel (from the left). The space occupied by the train is blanked by a rectangle. You can see the shock wave propagating inside the tunnel (red and yellow) and the wave radiating outdoors from the inlet (white and then blue on a green background ). A pressure wave also propagates from the back of the train (blue, yellow and red and then blue). Outside the tunnel, the simulation stops on the vertical plane of the tunnel entrance:
These are the pressure waves radiated from the tunnel ends when the train is travelling into the tunnel: