An advancing unit for positioning a current collector unit having at least one extension, which is guided in a fastening unit so that the extension can be moved linearly and which has a cavity, open on a longitudinal side extending in the advancing direction and the inner surface of which forms a contact surface for guiding elements arranged on the fastening unit. A flexible band is arranged on the extension. One section of the flexible band at least partially covers the open longitudinal side of the cavity along a section protruding from the fastening device, and another section of the flexible band is located in a reservoir. A first end of the band is connected to the fastening unit or to an end of the extension and the second end is supported on a tensioning device under a tensile stress in the longitudinal direction, which tensioning device, in the event of a motion of the extension by means of which the protruding section is shortened or lengthened, pulls the band into the reservoir or outputs the band from the reservoir by the same length and at the same velocity.