Dynamic blocking for railyards: Part I. Homogeneous traffic

This paper and its companion present a study of railroad classification yard strategies that allow for blocks of destinations to be assigned to classification tracks in different ways, depending on the time of day, week or month. With the same number of tracks, more classifications can be handled by this method. The paper examines homogeneous traffic; that is, traffic patterns where all blocks have the same amount of traffic, where cars for all blocks depart equally frequently from the yard and where the overall traffic flow does not change with time. The results represent the beginning of a better understanding of yard operations, which should be useful for designing new yards, planning expansions of existing ones and evaluating the impact of changes (planned and unplanned) on traffic patterns. The paper concentrates on two sorting strategies: sorting by train (perhaps the most commonly used strategy in the United States today), and triangular sorting. For both strategies, formulas are given for the minimum number of tracks, number of switches and for the total space requirements. For triangular sorting, the yard delay and total space depend on the time chosen between reswitches. These two measures of performance can be reduced if one is willing to accept a sorting effort slightly higher than minimum. The trade-off can be explored numerically. For sorting by train, yard delay and total space are not significantly affected by the sorting method; there is no such trade-off. It appears that sorting by train results in less work, delay and space requirements than triangular sorting, at least in most instances when either could be used. Triangular sorting, however, can be used when there are not enough tracks to allow sorting by train.