Abstract
This paper considers a Train Shunting problem which occurs in cargo train organizations: We have a locomotive travelling along a track segment and a collection of n cars, where each car has a source and a target. Whenever the train passes the source of a car, it needs to be added to the train, and on the target, the respective car needs to be removed. Any such operation at the end of the train incurs low shunting cost, but adding or removing truly in the interior requires a more complex shunting operation and thus yields high cost. The objective is to schedule the adding and removal of cars as to minimize the total cost. This problem can also be seen as a relaxed version of the wellknown List Update problem, which may be of independent interest.
We derive polynomial time algorithms for Train Shunting by reducing this problem to finding independent sets in bipartite graphs. This allows us to treat several variants of the problem in a generic way. Specifically, we obtain an algorithm with running time O(n^{5/2}) for the uniform case, where all low costs and all high costs are identical, respectively. Furthermore, for the nonuniform case we have running time of O(n^3). Both versions translate to a symmetric variant, where it is also allowed to add and remove cars at the front of the train at low cost. In addition, we formulate a dynamic program with running time O(n^4), which exploits the special structure of the graph. Although the running time is worse, it allows us to solve many extensions, e.g., prizecollection, economies of scale, and dependencies between consecutive stations.
BibTeX  Entry
@InProceedings{nonner_et_al:OASIcs:2012:3706,
author = {Tim Nonner and Alexander Souza},
title = {{Optimal Algorithms for Train Shunting and Relaxed List Update Problems}},
booktitle = {12th Workshop on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems},
pages = {97107},
series = {OpenAccess Series in Informatics (OASIcs)},
ISBN = {9783939897453},
ISSN = {21906807},
year = {2012},
volume = {25},
editor = {Daniel Delling and Leo Liberti},
publisher = {Schloss DagstuhlLeibnizZentrum fuer Informatik},
address = {Dagstuhl, Germany},
URL = {http://drops.dagstuhl.de/opus/volltexte/2012/3706},
URN = {urn:nbn:de:0030drops37066},
doi = {10.4230/OASIcs.ATMOS.2012.97},
annote = {Keywords: Train shunting, optimal algorithm, independent set, dynamic programming}
}
Keywords: 

Train shunting, optimal algorithm, independent set, dynamic programming 
Collection: 

12th Workshop on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems 
Issue Date: 

2012 
Date of publication: 

13.09.2012 