Stability of saturated timetables: the influence of buffer times
Résumé
In railway, the saturation problem consists in scheduling extra train paths within an existing timetable, intended to be a fixed reference. Inserting extra train paths reduces the global timetable stability, if compared to that of the not-saturated timetable. A compromise should therefore be sought, introducing constraints on the number and characteristics of the saturating paths in order to keep the stability of saturated timetables within a desired range. In this study, this task is performed by imposing that saturating paths are separated by a given buffer time. The study aims to provide some figures about the way in which buffer time affects the number of saturating paths and the stability of the resulting timetable. We saturate a real timetable (concerning a 70 km long French railway) using a mixed integer linear programming algorithm, imposing seven increasing values of buffer time. We perform the saturation considering two different time windows (during peak and off-peak hours) and two sets of saturating path types. The stability of the saturated timetables is evaluated through OpenTrack simulations with perturbed traffic conditions. The simulations are carried out with two perturbation scenarios. In this way the influence on stability of the parameters combination used in saturation is assessed. Results are presented and discussed, highlighting the role of the buffer time and of the type of saturating paths in providing stable saturated timetables.