In this work, the wavefront of a compression wave generated by a train moving into a tunnel is studied. A new one-dimensional flow model in which the flow around the train is considered to be a stream tube is proposed to determine the key parameters that affect the wavefront of the compression wave. From the result, the wavefront is found to be proportional to the lateral velocity at the height of the tunnel wall in steady flow around the train before the train enters the tunnel. This relationship between the wavefront and the velocity of the steady flow is validated from the experimental data. The result indicates that the wavefront of the compression wave can be predicted by investigating the steady flow field around the train in stead of the unsteady flow when the train moves into a tunnel.