Letter is an essential communication tool for any company to communicate with various parties. Although the modern era has been said to be characterized by a device or communication equipment more sophisticated, but the role of the letter can not be shifted away. It can be proved by the letters still widely used as a medium of communication today. Numbering can not be separated by making letter. Numbering has a function as the delivery of information that is important because in the numbering letter subject classification can identify letters, number maker unit and confidential letter or ordinary in the letter. This final project aims to determine the effectiveness and efficiency of using the numbering letter Rail Document System (RDS) on PT. Kereta Api Indonesia (Persero) Daop 4 Semarang.Data collection methods used were interviews, observation, and literature. Methods of data processing is was by identifying the handling or administration of numbering letter by observing the classification of the issues specified in a letter and giving the number of letters with Rail Document System (RDS). This research was conducted using the method of the description to explain the explain the effectiveness and efficiency of using the numbering letter Rail Document System (RDS) on PT. Kereta Api Indonesia (Persero) Daop 4 Semarang.