Reflective Nested Simulations Supporting Optimizations within Sequential Railway Traffic Simulators

Abstract

This article describes and discusses railway-traffic simulators that use reflective nested simulations. Such simulations support optimizations (decision-making) with a focus on the selection of the most suitable solution where selected types of traffic problems are present. This approach allows suspension of the ongoing main simulation at a given moment and, by using supportive nested simulations (working with an appropriate lookahead), assessment of the different acceptable solution variants for the problem encountered—that is, a what-if analysis is carried out. The variant that provides the best predicted operational results (based on a specific criterion) is then selected for continuing the suspended main simulation. The proposed procedures are associated, in particular, with the use of sequential simulators specifically developed for railway traffic simulations. Special attention is paid to parallel computations of replications both of the main simulation and of supportive nested simulations. The concept proposed, applicable to railway traffic modelling, has the following advantages. First, the solution variants for the existing traffic situation are analyzed with respect to the feasibility of direct monitoring and evaluation of the natural traffic indicators or the appropriate (multi-criterial) function. The indicator values compare the results obtained from the variants being tested. Second, the supporting nested simulations, which potentially use additional hierarchic nesting, can also include future occurrences of random effects (such as train delay), thereby enabling us to realistically assess future traffic in stochastic conditions. The guidelines presented (for exploiting nested simulations within application projects with time constraints) are illustrated on a simulation case study focusing on traffic assessment related to the track infrastructure of a passenger railway station. Nested simulations support decisions linked with dynamic assignments of platform tracks to delayed trains. The use of reflective nested simulations is appropriate particularly in situations in which a reasonable number of admissible variants are to be analyzed within decision-making problem solution. This method is applicable especially to the support of medium-term (tactical) and long-term (strategic) planning. Because of rather high computational and time demands, nested simulations are not recommended for solving short-term (operative) planning/control problems.

Dynamic Search of Train Shortest Routes Within Microscopic Traffic Simulators

“…Traffic simulations are widely used for rail traffic optimizations (simulation-based optimizations) with respect to specific criteria [2,3]. For example, one of the global traffic indicators that should be minimized is the sum of the weighted delays of all trains [4] within the railway network segment of interest. A simulation experiment requires many partial optimization problems to be solved during the run (such as the assignment of alternative platform tracks to delayed trains; determining the priorities for entering the line tracks if the timetable is not followed; searching for the shortest admissible routes for rail vehicle relocations within the rail infrastructure; appropriate location of the service resources within the infrastructure; etc.…”

“…The above-mentioned rail traffic simulations can be applied in conjunction with various specialized simulation tools, which can employ different rail infrastructure submodels [1,15,16,22]. Examples of relevant simulation tools used for such simulations include OpenTrack [23,24], RailSys [25,26,27], Villon [28], MesoRail [4], NEMO [29], PULSim [30], Simarail [31] etc. Such tools apply the same level of detail of investigation (microscopic, mesoscopic, or macroscopic) within one simulation model.…”

“…The extension of the research the results of which were published in [1,10] was driven by the need to broaden the modelling capabilities of the MesoRail simulation tool (briefly characterized in the supplementary material to the article [4]). This tool was initially used for mesoscopic rail traffic simulations only.…”

“…▪ The track infrastructure can be defined within a specific configuration file (in XML format), which uses templates inspired by the railML standard [35,36]. The infrastructure models can be constructed (e.g., for the MesoRail simulator [4]), within the TrackEd simulator [37] for instance. ▪ For the Shortest_Walk function (performing calculation within Algorithm 1), the finish relocation position ([ey,vg]∈F) within the track yard model (graph G) is represented by track/edge ey, reached through its particular end (vertex vg).…”

“…The track infrastructure models and shortest route searching algorithms described in the previous sections were implemented within the MesoRail simulator [4]. While initially designed for mesoscopic rail traffic simulations, this simulator was extended recently to be applicable to microscopic simulations as well.…”