The decarbonization of freight transport is currently a big challenge to tackle. One way of decarbonizing the sector is the modal shift towards the least polluting transport modes. This research aims to shed light on modal shift time dynamics in developing economies and considers whether a System Dynamics approach can assist with the policy-making decision about a modal shift towards freight decarbonization. This research explores policies that promote the modal shift of freight transportation for a Brazilian case study, using a System Dynamics model. Policies include fiscal and regulatory measures and infrastructure investments. The findings show that the process of modal shift is slow. However, implementing a combination of stricter policy measures early on, and changes in infrastructure investment strategies, accelerate the shift and this seems to be a robust measure package, capable of promoting a modal shift and decarbonizing the system. The model used highlighted how the system tends to adjust to modal shift measures, which tend to lose efficiency over time, slowing down the pace of decarbonization. Findings also display how modal shift policies alone might not be sufficient to achieve a reduction in CO2 emission. Addressing the problem with a System Dynamics approach may help decision-makers in economically developing countries to develop more effective policy strategies.