Pigs (10) are propelled forwards by a propulsion fluid in a pipeline (21). The pigs (10) have an upstream end (13), a downstream end (11), and an intermediate section (12). The intermediate section has a variable geometry chamber (16) associated therewith and one or more outwardly extending external pawl teeth (15). The pawl teeth are automatically displaceable outwardly relative to a longitudinal axis (9) in response to a pressure differential between a downstream pressure and an upstream pressure. The pig is configured such that a change in a geometry of the chamber in response to the pressure differential between the downstream pressure and the upstream pressure causes the one or more pawl teeth to automatically displace outwardly when the upstream pressure is less than the downstream pressure. When the pawl teeth are deployed, upstream displacement of the pig is inhibited. Various related systems (5) and methods are also disclosed.