A positioning drive (3) for positioning tamping tines of a tamping unit (1) has the following features: a) an inner cylinder (10) of the vibration exciter (8) that is arranged coaxially with the cylinder axis (7) is mounted in an outer cylinder (12) so as to be displaceable via sliding surfaces (11) in an axial direction (7), b) an annular depression (13) is provided between two sliding surfaces (11) spaced from one another in the axial direction (7), c) a spacer ring (15) which subdivides the annular depression (13) into two fluid ducts (14) which are spaced from one another with respect to the axial direction (7) is optionally connected to the inner or the outer cylinder (10, 12), d) for vibration excitation, the two fluid ducts (14) can be alternately charged with hydraulic pressure by means of supply lines (16).