To provide “comfortable space,” “mechanism for creating a quiet passenger cabin,” “mechanism to clean the air inside the car,” and “mechanism to prevent earache in tunnels” are required. When high-speed railcars pass through a tunnel, the air inside the railcar is drawn outward, and the pressure inside the railcar is reduced, causing an earache. High-speed railcars like the Shinkansen EMU have airtight car bodies to prevent this. On the other hand, ventilation is essential to keep the air in the room fresh. The difficulty of these issues lies in achieving these conflicting requirements. If the air pressure inside the car is kept constant, the car body will repeatedly expand and contract due to changes in the air pressure outside the car in a tunnel. They are designed to withstand that force. Aircraft are subject to the same air pressure changes, but there are some differences between aircraft and railcars. Railways provide mass transportation capacity and make it convenient to get on at any time of the day. The “coupling mechanism” that attains this will also be mentioned.