Achieving a socially and environmentally sustainable mobility and transport system necessitates a multifaceted approach that considers just Earth System Boundaries. Just Earth System Boundaries are domain-specific (e.g. climate change, water) thresholds beyond which significant harm is done to people and other species. We have crossed these thresholds in 7/8 domains and not yet met the minimum needs of people worldwide. The challenge is to return to the safe and just corridor while prioritising the access of the poorest people to minimum resources as called for by the principle of leaving no one behind. Within this context, the transport sector, a major contributor to climate change and environmental pollution, requires significant and swift transformations. This comment proposes six key principles for building a sustainable transport system: prioritising equitable access, enhancing public transport and limiting private transport, decarbonising fuel and fleets, decoupling freight transport from fossil fuel trade, repurposing infrastructure, and ensuring just financing. These principles may enable just living within just Earth System Boundaries.