Title

Integrating Dynamic Signaling Commands Under Fixed-Block Signaling Systems into Train Dispatching Optimization Problems

Document Type

Journal Article

Publication Date

2018

Subject Area

mode - rail, literature review - literature review, operations - coordination, infrastructure - traffic signals, planning - integration

Keywords

railway operations, signals, safety

Abstract

In railway operations, when a disruption occurs, train dispatchers aim to adjust the affected schedule and to minimize negative consequences during and after the disruption. As one of the most important components of the railway system, railway signals are used to guarantee the safety of train services. We study the train dispatching problem with consideration of railway signaling commands under the fixed-block signaling system. In such a system, signaling commands dynamically depend on the movement of the preceding trains in the network. We clarify the impact of the signaling commands on train schedules, which has so far been neglected in the literature on railway train dispatching, and we propose an innovative set of signaling constraints to describe the impact. The determination of the signal indicators is presented using “if-then” constraints, which are further transformed into linear inequalities by applying two transformation properties. Activation of the train speed limits that result from the signaling commands is the core purpose of the signaling constraints, and this is implemented by using the signal indicators. Moreover, we formulate the Greenwave (GW) policy, which requires that trains always proceed under green signals, and we further investigate the impact of the GW policy on delays. In numerical experiments, the proposed signaling constraints are employed within a time-instant optimization problem, which is a mixed-integer linear programming (MILP) problem. The experimental results demonstrate the effectiveness of the proposed signaling constraints and show the impact of the signaling commands and GW policy on the train dispatching solution.

Railways are crucial to address the ever-increasing mobility of people and goods, due to their positive characteristics of high capacity, high speeds, and eco-friendliness. A negative characteristic of railway services, however, is their limited reliability and punctuality, which hinders attainment of the modal share sought by policy makers and researchers. Train movements on a railway network are regulated by timetables. In daily operations, perturbations (caused by bad weather, infrastructure failures, extra passenger flows, etc.) unavoidably occur, which may affect the normal operations and cause a deviation from the planned timetable. In such cases, the task of a train dispatcher (controller) is to take proper measures to adjust the schedules affected, in order to reduce the negative consequences (delays). This is the train dispatching problem. Due to the high interdependency between trains utilizing the available capacity, train dispatching work is usually complex, especially when the railway network is operated close to saturation, in densely urbanized zones, or during peak hours. An ineffective train dispatching decision could result in a snowball effect with consecutive delays, downgrading the reliability and punctuality of train services. Fast and effective decisions for the train dispatching problem are always desired.

Railway signaling is one of the most important parts of the railway system. There is a wide variety of railway signals and many signaling systems with different principles all over the world, for example, the moving-block signaling systems commonly used in high-speed railway networks and the fixed-block signaling systems commonly used by conventional railways. The core function of the signaling systems is same, however—indicating the state of the block section(s) ahead for the train driver in order to guarantee the safety of train services. A signaling command can be indicated by a single aspect or by multiple aspects. In the United States and in many European countries, the signaling command provides an additional requirement for train operations, namely indicating the maximum allowed speed to the driver. The driver has to control the train to prevent it from exceeding this speed to ensure safety. Otherwise, a worse situation may happen, where the available distance is not enough to stop the train.

An extensive body of studies is available in the literature that addresses the train dispatching problem, having different focuses, such as: considering multiple classes of running traffic (1), passenger connections (2), speed management (3), and maintenance plans (4), and using different approaches (e.g., linear/nonlinear optimization and heuristics). However, a gap still exists with regard to the signaling commands. Train speed limits that result from signaling commands have been neglected in the literature (5), although they are indeed required in real train operations. To the best of our knowledge, no study is yet available for generating optimal train dispatching solutions that integrate precisely the actual signal aspect shown to train drivers, and that guarantee no violation of the signaling commands, based on the fixed-block signaling system. The reality of train operations and the gap in the scientific literature have motivated us to include the signaling commands while addressing the train dispatching problem.

We therefore study the train dispatching problem with consideration of railway signaling commands, focusing on railway networks with a fixed-block signaling system, as is common in the United States and in many European countries. As the signaling commands dynamically depend on the movement of the preceding trains in the network, we use binary variables (namely signal indicators) to indicate the signaling commands. These signal indicators are determined by a set of “if-then” constraints, which could be further transformed into linear inequalities by applying two transformation properties. These constraints could be generalized to other signaling systems. Train speed limits that result from the signaling commands are restricted in the signaling constraints by employing the signal indicators. In addition, we formulate the Greenwave (GW) policy and explore its impact on the train dispatching solution (i.e., the train delays). Basically, the aim of the GW policy is to require trains to follow their scheduled speed profile exactly, thus avoiding the need for speed profile adjustments. In the numerical experiments, a time-instant optimization approach (mixed-integer linear programming, MILP) proposed in our previous work (6) is used to apply the proposed signaling constraints, aiming at delivering a train dispatching solution with minimization of train delays. The experimental results demonstrate the effectiveness of the proposed model, including signaling constraints, and show the impact of the signaling commands and GW policy on the train dispatching solution.

Rights

Permission to publish the abstract has been given by SAGE, copyright remains with them.

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