CREWS_NS

First, besides dealing with time constraints and other constraints of the job shop domain (for example, equipment constraints), crew planners must also deal with space constraints to prevent space discontinuities in duties, positioning crew where they are needed, whether as passengers in trains or other transportation means. Second, crew planners must also deal with complex train frequencies, such as week frequencies (for example, a train might only run on weekends), year periods (for example, only during summer), and special days (for example, on holidays and days before holidays), which put additional constraints on the combinations of tasks. These two aspects are critical to the quality of the final schedules and the efficiency of the scheduling process, requiring abstraction techniques to be used extensively. Third, the work periods of crew do not have fixed times, as shifts in industry, but can slide during the day to accommodate the irregularity of the train operation, although subject to constraints. The resource-sliding dynamics make it difficult to analyze activity demand and resource contention as is usually done in the job shop domain (Sadeh and Fox 1991).