This paper quantitatively assesses the negative impact of land discontinuity on the development of a railway network on an island. This implicit cost of insularity is because an insular railway network only serves the territory in which it is located while the same network on a mainland also serves other regions. We apply this idea to the case of a simplified Italian railway network and we implement it through a simulation model. The simulation results highlight the strong negative effect of land discontinuity: whereas the railway lines located on the island of Sardinia are the least profitable under the factual scenario, their relative profitability is significantly boosted in every counterfactual scenario where the land discontinuity is artificially removed.