This was called a “tablet case” and was used to ensure safe train operation within single-track sections. Trains must pick up a tablet at the previous signal station within a section, proceed and return it at the next signal station, and if this tablet is removed, it will prevent another train from entering the section, resulting in a collision. It played a role in preventing accidents. In Japan during the Japanese National Railways era, it was commonly used in single-track railway areas, but in modern times more advanced signal control systems and communication technology have been introduced, and the use of tablet systems has decreased, making it extremely rare.