High-Speed trains entering tunnel produce a system of complicated wave phenomena. A strong compression wave at the entrance is formed, which then propagates ahead of the train at about speed of sound. This exits at the far end of the tunnel as an impulse wave creating a distinct aural discomfort and also provokes mechanical stresses on train cars triggering uneasiness to passengers inside it. Therefore, a study to understand the formation process of the very first of the compression waves produced at the entrance of the tunnel is of prime importance. To understand this phenomenon, a 3-D compressible, unsteady computational investigation has been conducted. The numerical scheme is validated against a reduced scale experimental study. The pressure being measured at two different locations inside the tunnel is used to study the changes in compression waves. The changes are comprehended using pressure contour plots and velocity vectors.