Transilien, the branch of SNCF in charge of operating the main urban railroad network in the area of Paris, faces a regular increase of passengers flows. The planning of railway operations is made carefully: simulation runs permit to assess the timetable stability. However, many disturbance 5 appears and cause trains delays. Due to the nature of the railroad network those delays are cumulative and an on-line update of the timetable is not always successful in maintaining the trains schedule. In this tensed context, operators are searching solutions to better use the infrastructure capacity and better service quality. A needed step towards this objective is a better understanding of the phenomena of disruptions. In particular because the expansion of congestion is not clearly 10 understood until now. This paper explores the possibility to transpose a traffic flow theory tool, the network fundamental diagram, in the field of dense railroad traffic. Railroad traffic is different of road traffic by many ways: railways are a planned system, traffic volume does not satisfy the continuum hypothesis, stations force stops and the signalization system brings a discrete behavior. Despite those big differences we show how to build a network fundamental diagram for a railroad 15 system and how to interpret some obtained shapes for those diagrams. These diagrams gives us some means to compare planned and reality. We also identify the limits that need to be overcome to take benefits of the road traffic tools in railroad traffic analysis.