When a railway train enters a tunnel portal at high speed, the air surrounding the train body is compressed due to the astrict of the tunnel wall. Additionally, pressure waves are generated and move along the tunnel at the speed of sound.

When a train enters a tunnel at high speed, it acts like a piston in a cylinder, compressing the air ahead of it. This phenomenon, known as the piston effect, creates a pressure wave that travels through the tunnel at the speed of sound. For passengers, this sudden change in air pressure often causes "ear-popping" or aural discomfort as the eardrum reacts to the imbalance. In extreme cases, particularly with high-speed rail, this pressure wave can emerge at the far end of the tunnel as a loud "thud" or sonic boom (micro-pressure wave) that can disturb residents and even damage nearby structures. To mitigate this, modern high-speed trains like the Japanese Shinkansen are designed with long, needle-like noses to "pierce" the air more gradually, and many tunnels now feature "hoods" or perforated extensions at the entrance to bleed off the pressure before it can build into a shockwave.