Maximizing Operating Reliability in Design of Long Single-Track Light Rail Transit Lines

At 79 km, the proposed Southeastern Pennsylvania Transportation Authority Cross County Metro would be the longest light rail transit (LRT) line in North America. The system would use an existing single track constructed on embankment, and adding a second LRT track over the entire distance would be prohibitively expensive. However, operating a single-track line of this length at 15-min headways would pose an unprecedented operational challenge using conventional approaches to siding location. The longer the line, the greater the likelihood that a late train will delay opposing trains and cause further delays. A methodology for locating passing sidings on single-track lines to mitigate the problem of cascading delays, even on long lines, is proposed. Unlike the conventional practice of locating sidings at the midpoint of scheduled meets, the sidings near the outer ends of the line are located off-center relative to the meet point, and are longer than those in the middle of the line. This tailors the length of siding cushion on either side of the meet point to match the risk of delay, and maximizes the effectiveness of the full length of each siding. Simulation results indicate that long single-track lines can be designed successfully to avoid delay propagation. However, the siding location methodology leaves little room for compromise. In addition, the resulting sidings are forever wedded to a specific operating plan. Station stopping patterns and operating speed cannot be changed once the sidings are in place, with headways only in multiples of the design headway.