A container joining system comprising: a first container and a second container, each of said containers (1, 2) being shaped as a rectangular prismatic polyhedron lying down, with two larger flanks, a roof and a base and two smaller flanks called front flank and rear flank, respectively, said first container comprising a door at its front flank and a door at its rear flank, and said second container comprising a door at least at its rear flank, said containers (1, 2) being suitable for being joined back to back by their rear flanks; four lockings for joining said containers (1, 2) back to back at four corners of their rear flanks, so that a space is leaved between said rear flanks of the two containers (1, 2) joined back to back; characterized in that it further comprises: a plurality of plates (4, 5) for closing from the inside of said containers (1, 2) said space along the entire perimeter of the rear flanks of said containers (1, 2), said plates (4, 5) comprising means for fixing them to inner sides of said containers (1, 2) and a seal provided in a portion of said plates (4, 5) that comes in contact with the inner sides of said containers (1, 2); displaying means for displaying an identification number, said displaying means being provided in each of said containers (1, 2) and comprising an electronic screen (101, 102) that displays the identification number, a control unit (102, 202) and a power supply; adjusting means for automatically adjusting the identification number displayed by said electronic screens (101, 102) when it is detected that the two containers (1, 2) have been connected or disconnected, said adjusting means comprising communication means between said control units (102, 202) and a contact (103, 203) provided in the position area of one of said plates (4, 5), such that the mere placing or separation of said plate (4, 5) is detected by said contact (103, 203) and said control units (102, 202) are automatically informed on the need to change said identification number. a first container and a second container, each of said containers (1, 2) being shaped as a rectangular prismatic polyhedron lying down, with two larger flanks, a roof and a base and two smaller flanks called front flank and rear flank, respectively, said first container comprising a door at its front flank and a door at its rear flank, and said second container comprising a door at least at its rear flank, said containers (1, 2) being suitable for being joined back to back by their rear flanks; four lockings for joining said containers (1, 2) back to back at four corners of their rear flanks, so that a space is leaved between said rear flanks of the two containers (1, 2) joined back to back; characterized in that it further comprises: a plurality of plates (4, 5) for closing from the inside of said containers (1, 2) said space along the entire perimeter of the rear flanks of said containers (1, 2), said plates (4, 5) comprising means for fixing them to inner sides of said containers (1, 2) and a seal provided in a portion of said plates (4, 5) that comes in contact with the inner sides of said containers (1, 2); displaying means for displaying an identification number, said displaying means being provided in each of said containers (1, 2) and comprising an electronic screen (101, 102) that displays the identification number, a control unit (102, 202) and a power supply; adjusting means for automatically adjusting the identification number displayed by said electronic screens (101, 102) when it is detected that the two containers (1, 2) have been connected or disconnected, said adjusting means comprising communication means between said control units (102, 202) and a contact (103, 203) provided in the position area of one of said plates (4, 5), such that the mere placing or separation of said plate (4, 5) is detected by said contact (103, 203) and said control units (102, 202) are automatically informed on the need to change said identification number. a plurality of plates (4, 5) for closing from the inside of said containers (1, 2) said space along the entire perimeter of the rear flanks of said containers (1, 2), said plates (4, 5) comprising means for fixing them to inner sides of said containers (1, 2) and a seal provided in a portion of said plates (4, 5) that comes in contact with the inner sides of said containers (1, 2); displaying means for displaying an identification number, said displaying means being provided in each of said containers (1, 2) and comprising an electronic screen (101, 102) that displays the identification number, a control unit (102, 202) and a power supply; adjusting means for automatically adjusting the identification number displayed by said electronic screens (101, 102) when it is detected that the two containers (1, 2) have been connected or disconnected, said adjusting means comprising communication means between said control units (102, 202) and a contact (103, 203) provided in the position area of one of said plates (4, 5), such that the mere placing or separation of said plate (4, 5) is detected by said contact (103, 203) and said control units (102, 202) are automatically informed on the need to change said identification number.