Abstract: Method and device for control within a line network such as a tram line, of a number of rolling units in various line runnings, and a method for installation of said device. A number of passive position determination elements (9) such as transponders, arranged for radio scanning, are located at determined positions in the network. Control equipment (2) on board said rolling units is provided with devices (8) for scanning of the position determination elements (9) and with sensors (6) for measurement of distance travelled. The momentary unit position within the line network is determined by continuous measurement of distance travelled and by calibration of the thereby obtained position determination by scanning of successively passed position determination elements (9). Data on the design and topography of the line network is stored in a central equipment (1) and communication from the mobile units stating their positions is received in a data processing unit (4).