Very few modeling elements:
Inventory flow problem
buckets of sand analogy – solves for how many buckets move to support desired flow of sand
Passengers arrive and depart at stations; can flow freely through the network provided vehicles are there to carry them.
Segments indicate time and not distance; transit graphs do not indicate geophysical layout of network
Multimodal: each vehicle type gets a completely new transit layer and network
Different size vehicles
Separate tracks/roads
Different operating costs