Transilien, the SNCF branch in charge of operating the main urban railroad network in the areaof Paris, faces a regular increase of passengers flows. The planning of railway operations is madecarefully: simulation runs permit to assess the timetable stability. However, many disturbances appear and cause train delays. Due to the nature of the railroad network those delays are cumulativeand 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 capacityand better service quality. A needed step towards this objective is a better understanding of thephenomena of disruptions. In particular because the expansion of congestion is so far not clearly understood. This paper explores the possibility to transpose a traffic flow theory tool, the networkfundamental diagram, in the field of dense railroad traffic. Railroad traffic is different from roadtraffic in many ways: railways are a planned system, traffic volume does not satisfy the continuumhypothesis, stations force stops and the signalization system brings a discrete behavior. Despitethose big differences we show how to build a similar tool for a railroad system, the Line Fon damental Diagram (LFD), and how to interpret some obtained shapes for those diagrams. Thesediagrams give us some means to compare planned and reality. We also identify the limits that mustto be overcome to take benefits of the road traffic tools in railroad traffic analysis.
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