An analysis of double-track railway line capacity

In Sweden, rail traffic is almost never separated according to speed. On several double-track lines the mix of heavy freight, regional and high speed trains imposes severe capacity problems. In order to evaluate the capacity for different traffic mixes, a combinatorial model -- Timetable Variant Evaluation Model (TVEM) -- has been developed. In this model both infrastructure and timetable are modelled as variables. Traffic is divided into train patterns according to a presumed regular timetable and then scheduled systematically in different time locations. The timetable variants are evaluated with regard to: mean values of capacity that give the number of trains/hr for the required mix, variance measures that show how the capacity depends on the timetable and scheduled delays that show the extension of run times imposed by overtaking. The paper shows how the important distance between adjacent overtaking stations can be sampled from Weibull distributions. TVEM has been used to evaluate three different operational cases with mixed traffic. The analysis shows that the impact on capacity from the infrastructure increases with speed difference and frequency of operation for passenger trains, while the importance of the infrastructure decreases when traffic is more heterogeneous. The impact from the timetable is strongest when the speed differences are low and/or the frequency of passenger trains is low. Capacity loss due to increased speed differences can be compensated for by additional overtaking stations. The slower trains suffer from a considerable increase in scheduled delays when speed differences increase.