Network = Graph
Public transport requires a fixed network of stops and connecting paths. From a mathematical point of view, you are dealing with a directed graph, i. e. every connecting path is represented by two edges – one for the outward journey and one for the opposite direction. A single-track rail connection would still have two edges1. The natural way to represent a transport network is to regard the stops as nodes and the connecting paths as edges in the graph. However, you can also choose the opposite representation.2
Incremental network planning
In Germany and many other European countries, network planning only takes place incrementally nowadays, i. e. when modifying and expanding the transport network, ...