Leveraging on the recent availability of open data about public transports, the last generation of smartphone applications provide highly personalised guidance to passengers during their trips. This smart assistance definitely improves the passenger comfort and streamlines their trips, especially in case of infrastructure incidents and/or multimodal trips. However, there are important limitations stemming from the unidirectional flow of information going from transport operators to passengers: (1) waste of computing resources, partially defeating the purpose of sustainability, and (2) missed opportunities of optimisations by the transport operators, which do not exploit detailed real-time passengers information. This paper presents ongoing work towards smarter and more sustainable multimodal transports based on a full-duplex ecosystem in which passengers and transport operators actively exchange information and react correspondingly. As first steps in this direction, we show how this integration can lead to greener computing applications by varying the balance between the smartphone and the cloud, and present a few concrete optimisations enabled in this model, during the trip itself or on a longer term by improving the transport infrastructure. We illustrate this ecosystem with a smartphone/cloud application prototype, and elaborate the remaining challenges for fully implementing this vision, including issues like interoperability, scalability, and acceptability.