Interoperability (and the related term interconnectivity) are core themes in the development of a Common Transport Policy (CTP) for the European Union (EU). This paper first investigates an expanded definition of interoperability that offers causal links to system organisation. A general model of transport system organisation in the context of this more specific definition is then considered. A number of generic policy changes are identified, which have the potential to bring about changes in transport systems that remove impediments to interoperability. With a better understanding of the causal links between system operation and impediments to interoperability, policies can specifically target a dimension of interoperability and a particular attribute of the transport system.