That's a very general question, thus I can only try to answer it in a similar general way: graphs with nodes and edges (or arcs, in the directed case) are a natural representations of street or railway networks with roads or tracks and crossings. Usually, weighted graphs are used, since the street are described with further data, such as a length or a travel time. Now several combinatorial graph algorithms can be used to answer natural questions about this graph. Most prominently might be: "What's the shortest distance between two nodes?" - This is what is the core of any car navigation system and also internet based route planning tools (such as google maps, and many others). If there are not only two places to be linkes, but maybe a round tour through several places, this leads to the study of the traveling salesman problem. If one is interested in delivering mail or cleaning the streets, so that all streets must be traversed at least once, then we're asking for a solution for a problem known as the Chinese Postman Problem (because it was introduced by a Chinese mathematician). In many applications, there is more then just one vehicle, so it is necessary to study the "multiple" variant of these problems, when besides the routing there is an additional assignment component involved. Then there could be considerations of time-windows, load (pickup-and-delivery), ...

I hope that could give you a very first orientation. To each topic mentioned above, there is a vast amount of literature. If you are interested in one of these topics in particular, then please just post a follow-up question.

Mobility management is also used as a term to study - in a transportation network context - how traffic (as in work-related or recreational trips, delivery of goods (lorrys) or people (bus, taxi, bicycles, ...), etcetera) can be influenced to make the total transportation "work" become more sustainable with regards to both environmental and economic factors. It includes information-based services such as information about which routes to take to travel the fastest to given destinations, and actual re-building of the traffic network to make cycling more accessible (typically at the cost of car accessibility). In my town of Gothenburg we have gotten several roads in the centre where cars and bicycles share a lane but cars are forbidden to travel faster (that is, no overtaking is allowed), hence giving bicycles priority. Car pooling could be considered as such a system, as fewer cars are then needed among the dwellers in a neighborhood having a car pooling system.