Inspired by the growth dynamics of the protist Physarum polycephalum, we apply the formalism describing adaptive incompressible Hagen-Poiseuilly flows on channel networks to find graphs connecting different nodes distributed on an Euclidean space. These graphs are suboptimal or optimal relative to the graph length. We sometimes obtain graph tree configurations topologically equivalent to Steiner trees. This approach is applied to design communication systems, such as railway systems, highways, or fibre webs. As a proof of concept, we explicitly apply this framework to the Portuguese railway systems.