Cilia assembly and function require intraflagellar transport (IFT), a mechanism that uses “trains” to transport cargoes into and out of cilia. While much has been learned about IFT in the past decades, IFT train assembly, loading of cargo and transport regulation have remained poorly understood. In a recent study, Hesketh, Mukhopadhyay and colleagues obtained the complete structure of the IFT-A complex, a key element of IFT trains. By modelling IFT-A into anterograde trains and performing structure-guided mutagenesis, the authors uncover how the IFT-A complex polymerizes and forms carriages to accomplish its distinctive functions.