The theoretical and practical research on using the extra capacity of rail transit passenger lines to provide freight services is in the ascendant, which can be divided into two modes: adding dedicated freight trains into passenger trains timetable and using the residual space on passenger trains to fix freight transportation. At present, the theoretical research mostly focuses on the optimization of mixed passenger and freight transportation scheme and timetable, and the practices mostly concern how to fix small freight demand using the residual space on passenger trains. Based on the empirical cases of Japan's North Vietnam express line, high-speed rail express, Taoyuan airport MRT and German parcel intercity (PIC), this paper summarizes the characteristics of two transportation organization modes. The organizational scheme of using the residual space on passenger trains to fix freight transportation is mainly used to meet the scattered freight demand. When using the carriage space, necessary fixed facilities shall be provided to ensure safety. Separation of transportation and loading can improve the loading and unloading efficiency and reduce the interference to passenger transport operations. In the case of adding dedicated freight trains into passenger trains timetable, the speed of the freight trains should be close to the adopted passenger train speed, and the transport process organized at night can ensure timeliness.