Network topology
A metro network's physical topology consists of rail links and rail stations, which is the basic constraint for passenger route choices in the network. With the developments of metro systems, the network's expansion has enabled more flexible and complex travels within the metro system because more transfer stations are providing better connecting services. In other words, in a metro network, there are different connecting routes for a passenger to choose for a given origin–destination (O–D) pair. The physical topology based search, consequently, results in the generation of a universal set of routes, which serves as the initial route set and is filtered by other constraints such as travel cost, train operational plan, etc.