One of the first models of a traffic flow was created by Jain  in his packet train model. He defines a packet train as a burst of packets arriving from the same source and heading to the same destination. If the spacing between two packets exceeds some inter-train gap, they are said to belong to different trains. In his model, the intertrain time is a user parameter, dependent on the frequence with which applications use the network. The intercar arrival for a train is a system parameter and depends on the network hardware and software.
This model reflects the fact that much of the communication inside a network involves in fact many packets spaced closely in time that are exchanged between the same two endpoints.