Editorial for S&C benchmark special issue

Switches and Crossings (S&C, turnouts) constitute key components in railway networks as they allow for trains to change track. This functionality comes at a cost as the rail discontinuities in switches and crossings induce greater loading and faster degradation compared to plain line. S&C are frequently characterised as ‘hungry assets’ with maintenance costs equivalent to about 0.3 km of plain line track [1]. It is therefore clear that modelling and simulation efforts that can further the understanding of S&C performance and allow for design optimisation is extremely valuable in a stressed economical context.

The railway dynamics community has been relatively late to arrive at the topic of dynamic vehicle-S&C interaction. There are relatively few publications before the turn of the century [2–10], but they have grown significantly in number and in technicality since. Starting with just the challenge of simulating the dynamics of a vehicle negotiating S&C and seeking to establish general lateral forces and safety criteria, efforts are now turning towards damage prediction and optimisation. Given the level of maturity reached in this field, it was considered that a Benchmark exercise would be highly important to document the different modelling approaches and capture the state-of-the-art at this important phase in the development of the subject area.

S&C merit the attention of a tailored Benchmark as they constitute some specific challenges in terms of modelling and simulation of dynamic vehicle-track interaction. In S&C, there are large and sudden changes in rail profile geometry along the track. This constitutes a challenge in terms of rail surface modelling for the contact algorithms. Given the stiffness of the wheel–rail contact, the slightest distortion in the rail surface description can induce a significant shift in contact conditions and result in large dynamic contributions to the contact forces. It is also the case that wheels passing through a switch or a crossing panel can make simultaneous contact with multiple rails that can deform relative to one another, for example the stock rail and switch rail in the switch panel and the check rail and stock rail in the crossing panel. This calls for more elaborate track and wheel–rail contact modelling compared to plain line. Finally, track properties vary along an S&C by design, which is also a departure from plain line modelling assumptions of a simplified homogenous co-running track model onto which railway vehicle MBS were originally built on.

For this Benchmark, participants were given the task of modelling separately the switch panel and the crossing panel for two different S&C designs of different length and to simulate dynamic vehicle-track interaction in those panels [11]. Input data was provided in the form of track layout, rail discrete cross-sections at specific positions and the track properties were represented using co-running track models with specified properties for each panel. Traffic is represented by the passenger vehicle from the Manchester Benchmarks [12,13]. The focus of the Benchmark has been on establishing the general interaction forces while comparing the methods used to represent the varying rail shapes. It has been a pure simulation exercise where simulation results are compared to one-another and the kinematic motion of the wheels relative to the rails has been one of the keyways to verify and compare modelling approaches. In addition to simulation results, all participants were requested to submit a method statement describing how the simulation cases had been implemented.

The idea of an S&C simulation Benchmark was originally conceived by the authors during the Horizon 2020 Shift2Rail In2Rail project in 2017. It was announced at the IAVSD conference held in Gothenburg, Sweden, in August 2019. It was our pleasure to observe that it attracted significant interest from the railway dynamics community. In the end 18 participant groups submitted results using nine independent software tools. Among the software tools two are in-house codes while the others are commercially available.

The Benchmark was initially carried out blind by all participants. There was thereafter a period of consultation during which interim results were made available, leading to revision and improvements, mainly because of errors of interpretation or implementation. The purpose of this procedure was to try and bring out the best in each software and modelling approach and make sure that the end discrepancies in results were due to differences in modelling choices and software rather than errors or varying interpretations of the Benchmark cases. The Benchmark activity was finalised with a two-day workshop in the end of April 2020 where the final results and the methods used were compared and discussed. While we had looked forward to meeting everyone in person, the workshop had to be held online due to the ongoing covid-19 pandemic.

The main purpose of the results and method statements papers [14] stemming from the Benchmark is to present and compare the results submitted by participants in response to the Benchmark task together with an overview of the corresponding modelling and simulation techniques they used to complete it. A side-by-side comparison of the methods used in the key modelling domains for easier comparison was chosen over publishing individual method statement papers form each participant. All these method statements are available online for those that wish to see the full detail and specific software implementation. One of the key outcomes of this Benchmark was to allow new engineers the opportunity to start in this field with an established reference to work against. So, for those that wish to perform the Benchmark exercise in posterity, the description is available in the modelling description and simulation tasks paper [11] where one can also find links to the input data and reference results in numerical format.

The Benchmark motivated considerable exchange and led to collaborative additional work. Special contribution papers were invited and managed on the basis that they were (1) extending the scope and value of the Benchmark in areas that were not feasible to study within the Benchmark itself and were (2) collaborative in nature involving at least two different participants and comparing a range of software. A total of 13 original contribution papers were submitted to this special issue from a total of 42 individual authors representing 20 different institutions.

The themes explored spanned the entire system from vehicle, wheel-rail interaction and track. One major area of interest stimulating five papers focused on the improved modelling of the track system introducing advanced co-running track models dealing with detailed switch and stock rails relative movement [15], flexibility in track elements using beam on discrete support [16,17] to better capture higher frequency loads and track resistance and even introducing axle flexible modes and their influence on dynamics forces [18]. The other major area is around taking better account of variability in the track geometry [19] and introducing field measurement in the process, in particular for worn rails [20,21] to reflect how asset performance changes with time and wear. The abilities of various wheel-rail contact algorithms to deal with the complex situation of rapid shape changes is also further elaborated in [22] showing areas where contact model can be further developed in the future, conformal contacts and sharp edges in particular. Influential parameters from vehicle and operation [23] and wheel-rail contact friction [24] are further investigated and their influence established on the overall modelling. Finally holistic simulation schemes [25] making used of MBS simulation in loops calculation together with other damage models (track settlement, rail wear and plastic deformation) are proposed that can help understand and solve the long terms degradation of S&C.

As a final note, we are very hopeful that this collective work is a steppingstone for further research to develop in this field. In particular seeing the development of new technologies being applied to solving S&C operational and financial issues, and thus helping to maintain the railways being seen as part of the solution to the future of sustainable transportation.

We would like to thank the European Commission for partly funding this work within project In2Track (Grant agreement No 730841) and In2Track2 (Grant agreement No 826255) and UK EPSRC Track to the Future (Grant agreement No EP/M025276/1). In particular project partners Network Rail and Trafikverket (The Swedish Transport Administration) for their support in providing the necessary input data that made this Benchmark possible. We would like to thank all the authors for their contributions to this special issue, and their institutions for allowing them to contribute to this work. We also wish to thank the reviewers for their insights and recommendations that greatly helped the authors to enhance their contributions. Finally, we would like to thank the editor, Professor Simon Iwnicki for his guidance, and the journal’s editorial team for their commitment to the publication of this special issue and the IAVSD for they support throughout.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by Engineering and Physical Sciences Research Council [grant number EP/M025276/1]; European Commission [grant number 730841,826255].