Junction capacity and performance in rail transit

In this study, steady-state cyclical queueing methods are used to associate utilization rates of junctions to delays, and to determine queueing lengths and estimate delays entering a junction with automatically collected train location data. Capacity is determined for crossings and merge points in urban rail systems, which are characterized by high frequencies and short train lengths. Actual throughput is compared to the theoretical and practical junction capacities to analyze the factors that constrain capacity. A new configuration for the junction is examined based on these factors and suggestions are made where the new configuration has not addressed factors that affect the current capacity. The Clark Junction at the Chicago Transit Authority is presented as a case study. An analytical study of capacity shows that the junction can handle about 30 trains per hour at each crossing, and 36 trains per hour at the merge point with the current service line frequency ratios. However, during periods of high frequency the crossing of northbound Red Line and northbound Brown Line trains handles about 26 trains per hour, whereas the merge point of southbound Purple Line and Brown Line trains handles only 22 trains per hour. Thus, the junction is not operating at capacity on any of its approaches. The lower throughput is partly due to the dwell time activity of trains at the neighboring Belmont station. At the current throughput, Belmont station cannot process trains without delays on followers. During the morning peak period the highest congestion is experienced in the southbound tracks and there are on average 13 Brown and Purple Line trains delayed as they approach Belmont station. Queues of as many as three trains form at least once during the peak period on the Brown Line service with the resulting delay being more than 2 minutes. During the evening peak period the highest congestion is experienced in the northbound tracks, resulting in delays on almost a third of the Red Line trains as they approach Belmont station and delays on a fifth of the Brown and Purple Line trains approaching Belmont. On each of the approaches to Belmont, queues