Big pressure changes outside the train can have an effect in the carriages themselves, particularly where there's not much clearance between the top of the train and the roof of the tunnel. This is why your ears sometimes pop when travelling through tunnels at high speed.

Your ears pop on the Shinkansen due to rapid changes in air pressure, especially when the train enters or exits a tunnel at high speeds (often exceeding 300 km/h). As the train enters a confined tunnel, it acts like a piston in a syringe, compressing a massive column of air in front of it. This creates a "pressure wave" that pushes air into the train's ventilation system and through any small seals, increasing the pressure inside the cabin. Conversely, as the train moves through, the air rushing backward creates a low-pressure suction. This change in external pressure causes the air trapped behind your eardrum to push outward or inward against the eustachian tube. To combat this, modern Shinkansen trains are pressurized like airplanes to minimize the effect, but the sheer speed and the "piston effect" of Japan's many mountain tunnels mean that sensitive ears will still feel the "pop." Most travelers find relief by swallowing, yawning, or chewing gum to help equalize the pressure between their middle ear and the cabin environment.