Optimization Of Freight Train Length In The Market Competition

With the development of industrialization and transformation of consumption structure,the transportation demand structure has been evolving.Meanwhile,the competition among multiple transportation modes has been shifting from the cost competitive stage into the comprehensive service quality competitive stage.They all claim higher requirements regarding the timeliness and comprehensive service quality for the railway transportation.For a long time,the railway in our country adopts the train operation organization system.Most trains require transfer operations in a marshalling station,where cars assemble into a train and depart until there are enough cars available for the departure size limitation.Therefore,the delay time is difficult to predict and the goods delivery time cannot be guaranteed.This predicament has raised new demand for the operation mode in marshalling station.In consideration of the market competition,the optimization of the freight train length aims at seeking the optimal minimum number of cars by measuring train operating costs and benefits.When the number of assembled cars reaches the minimum constraint,the train could depart in the premise that it should not exceed the maximum constraint(i.e.the departure size limitation).It is a break from the rigidity constraint that the assembled car number must reach the maximum before the train departs.It could improve punctuality of scheduled railway service and timeliness of railway goods transportation,and also provide guarantees for the railway transportation adapting to development of modern logistic and improving comprehensive service quality.Therefore,this study aims at investigating the car assembly queuing system and evaluating the system performance firstly.Then,the optimal train length model has also been addressed to maximize the profit of the railway operators in multi transportation modes environment.This dissertation is structured as follows:This paper first teased out the definitions of train length,then provided the influence factors from the perspective of capacity,train diagram structure,economy,traffic demands,competition among multi transportation modes environment.The discrete time queue theory was also introduced,and a car assembly queuing system was described,which was fundament to investigate car assembly queuing models.Finally,research problem and scope of this study were also defined.The car accumulation process is an inevitable and time-consuming railway technical operation,and the train length is closely related to this delay time.In Chapter 3,this study investigated the case that when there are not enough cars available at a departure epoch,the system lost the train path.A batch-arrival and batch-service queuing model for arrival-assembly-departure of cars was built using the method of discrete time queuing theory.Queue length distributions at departure epochs were obtained based on the embedded Markov chain technique.Next,relationships between queue length distributions at arbitrary and departure epochs were derived using the method of supplemental variable.Then,mathematical expressions of various performance of interest were obtained,including the mean queue length,the mean accumulation delay of a car,efficiency,traffic volume in one day etc...And then though numerical examples,the impacts of minimum number of cars,batch size distributions,arriving intensity and service time distributions on the system performance were also analyzed.Finally,how the optimum minimum number of cars changes with the increasing arriving intensity were investigated.Chapter 4 extended the model proposed in Chapter 3.The extensions included the car assignment between adjacent departure trains into the model.A batch-arrival and batch-service queuing model was built for arrival-assembly-departure of cars.Queue length distributions at departure epochs were obtained based on the embedded Markov chain technique.Next,queue length distributions at arbitrary epochs were derived using the method of supplemental variable.Based on these distributions,mathematical expressions of various performance of interest were obtained,such as the mean queue length,the mean accumulation delay,efficiency,capacity utilization,traffic volume in one day.The impacts of car assignment on the system performance were investigated through some numerical examples by comparing the car assignment scenario with the no car assignment scenario.In Chapter 5,this study investigated the case that when at a departure epoch,there are not enough cars available,the system lost the scheduled train path but added a new one meanwhile.For the problem that cars assemble in a marshalling station in the time-fixed mode with soft terms,a discrete time queuing model with batch-arrival and batch-service was established.Queue length distributions at departure epochs were obtained based on the embedded Markov chain technique.Next,queue length distributions at arbitrary epochs were derived.Based on these distributions,mathematical expressions of various performance of interest were obtained,such as the mean queue length,the mean accumulation delay of a car,efficiency,and traffic volume in one day.And then the the impacts of minimum number of cars,batch size distributions,arriving intensity,service time distributions on the system were also analyzed though numerical examples.Changes of the optimum minimum number of cars with the increasing arriving intensity were also analyzed.Finally,comparisons with the model in Chapter 3 were also presented.Chapter 6 extended the model proposed in Chapter 5.The extensions included the car assignment between adjacent trains into the model.In the time-fixed assembly mode with softened terms,the earlier train leaves a part of cars to the later one in the premise that it satisfies the assembly end conditions,so that both trains could depart on schedule.To achieve this,a batch-arrival and batch-service queuing model for arrival-assembly-departure of cars was built.Queue length distributions at departure epochs were obtained based on the embedded Markov chain technique.Next,queue length distributions at arbitrary epochs were derived.Based on these distributions,mathematical expressions of various performance of interest were obtained,such as the mean queue length,the mean accumulation delay of a car,efficiency,capacity utilization,and traffic volume in one day.The impacts of car assignment on the system performance were investigated through some numerical examples by comparing the car assignment case with the no car assignment one.Chapter 7 considered the competition among multiple transportation modes.The generalized cost function and freight volumes were influenced by minimum number of cars.A bi-level programming model was proposed with the minimum number of cars as the decision variable.The lower level program presented a multi-mode equilibrium model with elastic demand,which could offer a railway freight volume.While the upper level aimed to maximize the benefit of railway operator.An iterative heuristic algorithm was developed to solve this model,while the lower level program was solved by excess method.