Operational Schedule Flexibility and Infrastructure Investment: Capacity Trade-Off on Single-Track Railways

Abstract

To minimize costs, railways carefully match route infrastructure investment to projected traffic levels. Under structured operations according to a rigid timetable, the optimal amount of infrastructure for a given traffic volume can be determined with various models. However, North American railways use flexible operations for which planned train departure times can vary. Schedule flexibility increases the number of possible meet locations on a single track, which can lead to a combination of increased delays and infrastructure investment. When industry practitioners plan rail line capacity for flexible operations, they rarely optimize schedule flexibility and infrastructure investment simultaneously for a given level of service. To increase knowledge of the relationship between these factors, the research reported in this paper simulated operations on two representative single-track routes with rail traffic controller software. A baseline minimum-delay schedule was developed for each route under an initial infrastructure. The experiment design introduced schedule flexibility and infrastructure expansion to examine the interaction between these factors and train delay response. The results suggested that, for a given infrastructure configuration, an immediate increase in train delay occurred with small amounts of schedule flexibility. After this increase, routes became insensitive to further increases in schedule flexibility. Similarly, to maintain a level of service required substantial infrastructure investment for small amounts of schedule flexibility.

Transitioning from flexible to structured heavy haul operations to expand the capacity of single-track shared corridors in North America

“…For a given traffic composition and level of schedule flexibility, the normalized average train delay output from the simulation scenarios exhibits the expected relationship with traffic volume (Figure 2(a)). Consistent with previous research, 21 at a constant traffic volume, increasing schedule flexibility leads to increases in train delay.

Figure 2.(a) Average train delay and traffic volume for different levels of schedule flexibility transformed into (b) line capacity for a given schedule flexibility and maximum allowable train delay.

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“…Previous research by Dick and Mussanov 21 indicated that when operating with schedule flexibility at a constant traffic volume, the incremental increase in train delay diminished as schedule flexibility reached higher levels (Figure 1(a)). On North American single-track mainlines, it is generally accepted that increasing traffic volume leads to exponential increases in freight train delay (Figure 1(b)).…”

“…Although some decrease in schedule flexibility may be achieved through improved operating discipline, as described by Dick and Mussanov, 21 reducing schedule flexibility and transitioning traffic to a structured operation to increase capacity may not be without cost. To reduce schedule flexibility caused by unavailability of resources, railway operators may need to invest in additional crew, locomotives, and rolling stock.…”

“…Zero minutes of schedule flexibility represent the structured operation on the baseline schedule, and 720 min defines fully random departures over each 24 h period. With previous research concluding that moving from medium to high levels of schedule flexibility does not substantially change train delay, 21 additional factor levels between 120 and 720 min of schedule flexibility were not included in the experimental design.…”

“…This paper builds upon a previous study of the relationship between train delay and different levels of schedule flexibility under homogeneous operations and a constant traffic volume on representative mainlines in North America. 21 Beginning with an initial timetable designed to minimize average train delay, introducing schedule flexibility by increasing the range of departure time variability caused average train delay to increase. The relationship between schedule flexibility and train delay was found to be nonlinear: low levels of schedule flexibility resulted in disproportionately large increases in train delay while, beyond a certain level of schedule flexibility, additional incremental train delay was not observed as schedule flexibility reached high levels.…”