University of Twente Student Theses


An optimization model for rail line crossover locations considering the cost of delay

Trommelen, W.W.T. (2020) An optimization model for rail line crossover locations considering the cost of delay.

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Abstract:Double track rail lines are often provided with crossovers. A crossover is a pair of two switches, making it possible to ride from the inbound track to the outbound track and vice versa. One of the functions of crossovers is the possibility for alternative schedules during disturbances on the rail line. Rail lines without rerouting options are often split up in two circuits during disruptions. This makes it possible to still use the non-disrupted track part during disruptions. To operate a shortened part of the line, a crossover is needed to turn back to the track in the right direction when turning to the other direction. Turning can be done beyond the last-to-reach station, without passengers. Another option is to turn at the station and change the switch while passengers get off and on. In that case, the tram is guided to the correct track after or before turning. Tram lines often do not have a crossover before and after every station. This means that a large part of the line is often unavailable during disruptions. Sometimes operators do this on purpose, they use buses to connect the stations in case of disruptions. However, this is not a realistic measure in all cases. Sometimes the bus routes are much longer than the rail line. Adding crossovers is a trade-off. Crossovers have high purchase and maintenance cost. Moreover, crossovers break down often, because they are vulnerable railway parts. Therefore, the delay benefits of crossovers are sometimes lower than the delay cost. In recent years, rail managers try to use as little as possible crossovers in their networks. They try to use the crossovers as effective as possible. Past works studied the trade-off topic of rail infra cost versus passenger impact as well. However, those works were only able to compare a few alternatives, because the degraded schedules had to be assigned manually. They concluded that passenger delay is a fair indicator for rail line performance, for passengers, operators and governments. There are no past works that developed an optimization problem for crossovers. In this thesis, this is done by minimizing passenger delay. The optimization model is set up for the location of crossovers for double track light rail lines. The model is specific for lines without rerouting options via another rail line in the network. The model minimizes the total monetized passenger delay cost, by modelling all possible disruption scenarios on each track segment. A track segment is a track part between two stations, between two crossovers or between a crossover and a stop. For the complete segment yields that the same degraded schedule is the best option. An algorithm is defined to determine the degraded operation schedule for these disruption scenarios. For each origin-destination pair (station to station on the case study line), the travel time during disruptions is calculated. The model also considers walking or another public transport line if that is quicker during the disruption. A set of potential crossover locations is defined, and the delay cost are calculated for all of these potential location combinations. To do this, all disruption scenarios with their probability and average duration are used. Analysis to the maximum potential crossover location set size is done, considering the computer computation time. A case study is used to determine the usability of the model outcome. The case study is a new tram line in Bergen (Norway). This line connects the city centre, a university, a hospital and some suburbs. Using busses in case of disruptions is not a realistic option here, because the tram line traverses two mountains without roads. The optimal design according to the model is compared to the actual design. This actual design is currently being constructed in Bergen. For each origin-destination pairs (station to station), there is analysed if the effect is positive or negative. The model is also compared to a crossover performance optimization model. This model counts the crossover usage, without taking passenger numbers and delay minutes into account. Key performance indicators from past works are used to compare the designs: crossover performance, delay minutes, connectivity during disruptions and the number of passengers delayed more than 5 minutes. Validation tests are done using random numbers for the disruption probability, average duration and number of passengers between all stations. The best design according to the delay minimization model seems robust according to these tests. In this design, travellers have 10% less delay on average during non-recurrent disruptions than with the real design. However, the assumptions and simplifications of the model could have influence on the delay minutes. They might be slightly higher in practice, because the transition phases and capacity of vehicles are neglected in this study.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Programme:Civil Engineering and Management MSc (60026)
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