A plunger (10) located in a pipeline (20), which can be used as a pump or for pigging but which allows the pipeline (20) to be used as a normal pipeline when the plunger (10) is not in use includes an axial passage (12) extending in the same direction as the pipeline (20), wherein the axial passage (12) includes a valve (16) which can be used to open or close the passage (12) through the plunger (10). Means are provided to move the plunger (10) along the pipeline (20), and these may take the form of a magnet (14) in the plunger (10) and an electrical stator (22) in the pipeline (20), such that supply of electrical power to the stator (22) can be controlled to control the speed and direction of movement of the plunger (10). Stops (24, 26) may be provided to limit the range of movement of the plunger (10) in the pipeline (20), and one of these stops (24, 26) may be associated with a one-way valve.