A network on an island only serves the territory in which it is located, while on a mainland region the same network would also serve other regions. This paper quantitatively assesses this effect through a model which simulates the construction of a railway network in Italy. The negative effect of land discontinuity on the development of an insular railway network is found to be quite strong: while the railway lines located in the island of Sardinia are the least profitable under the factual scenario, their relative profitability is significantly boosted in every counterfactual scenario where land discontinuity is artificially removed.