<p>The internalisation of sustainability-related external costs in the choice of the optimal mode of transport in the hinterland of ports is seen as key to increase the share of rail-based combined transport in the context of efforts to improve transport sustainability. We develop a distance-based transport cost model to determine the optimal modal split for container transport for two cost scenarios: the current economic transport costs and the situation with the internalisation of external costs. With the help of Monte Carlo simulation, the model makes it possible to investigate the effects of changes in the hinterland structure on the optimal modal split. We apply the model to the case of the North Adriatic ports. Our estimates of the optimal modal split show that for ports that have expanded their hinterland in the last ten years, it is already economical to rely mainly on combined transport. We suggest the main reason for the lower actual modal shift in these ports is the underdeveloped rail infrastructure, which as such is the main obstacle on the way to sustainable port-hinterland transport.</p>