Populations in cities are constantly increasing, which puts more stress on existing transportation systems and especially public transport lines. A good example of this is the situation in the Île-de-France (IdF) region around Paris. There, the Grand Paris Express (GPE), a new system of major rail-based public transport lines, is planned to be introduced to complement the existing offer by connecting the suburban areas of Paris without having to go through the center of the capital city. These new lines are expected to be put in service progressively from 2024 to 2030. Along with additional tram-based lines, they are expected to significantly reduce travel time for some parts of the population. In this work, we adopt an agent-based simulation approach in which a synthetic population of IdF with activity chains of is generated. The population is then fed to the simulation with the currently existing public transportation offer as well as the future lines that we consider. The impact of the new offer is evaluated on a subset of the IdF area. We find that future rail-based public transport lines will not only impact existing rail lines but also majorly impact the bus lines which are rendered less attractive in comparison. However, ridership of certain bus lines is greatly increased. Moreover, we assess the potential of using a Mobility-on-Demand feeder service to increase accessibility to rail-based modes. Results show that such a service increases the ridership of related transport lines, while bus trips are decreased. We measure the performance of the Mobility-on-Demand system under different fleet settings. This research is fully reproducible as we solely make use of fully open data, tools and methodologies. We identify development pathways that will be investigated further in future works for a more comprehensive assessment of future transportation systems in IdF.