Simplified Dynamic FEA Simulation for Post-Derailment Train-Behaviour Estimation through the Enhanced Input of Wheel–Ballast Friction Interactions
Round 1
Reviewer 1 Report
This study proposes a novel 3D simulation model of trains that copes with phenomena after derailment of trains for a purpose to improve safety of transportation systems, which will attract attention from readers in various fields of engineering.
In this study, wide range of factors that affect the behavior of the trains such as collisions between cars and deceleration due to friction between wheels and ballasts after the derailment were considered to obtain more general and accurate results. The numerical results were compared to an actual accident and successfully showed qualitatively good agreement.
Therefore, I think this manuscript can be published basically as it is. However, I propose following minor points to gain better understanding from the readers.
- Line 81, 125, 132, 156 and 187
Typos related to number of figures are found in those lines.
FYI: When you upload your manuscript in Word format through web site of MDPI, it is automatically translated to PDF format. It seems to cause these errors sometimes. In my experience, by manually translate the Word file to PDF file, the error did not appear.
- Line 161 and 169
Figure 2 may be Figure 7
- line 60
The title of the section 2 is Finite element model as same as that of section 3. it may be better to be another expression, for example, Simulation model.
- Line 89
In table 2, I think it is better to insert an empty line between data for EMU-260 and EMU-320 to improve readability.
- Line 205
In my opinion, you should mention value of the friction coefficient μB used in this simulation as well as a reason why the value was chosen for this case. In the previous section, it was shown that coefficient μB has significant effect on phenomenon of the train after the derailment, so mentioning the choice of this parameter will make the results more convincing.
- Line 215 and 217
In figure 10, an arrow to indicate distance between car no.4 and 5 is shown to be 200m. I think it is better to be shown as 50m. Correspondingly, it is better that the points at 150m from the derailment point and extra 50m are indicated by additional arrows in figure 11 for easier comparison to figure 10. It will give impression to the readers that the simulation result demonstrates good agreement with the actual accident.
Thank you for your attention.
Best regards,
Author Response
Response to Reviewer 1 Comments
I'm attaching a paper that has been revised for your comments, and I've written a response to your comments at the end of the file.
Please see the attachment files.
Point 1: Typos related to number of figures are found in those lines. (Line 81, 125, 132, 156 and 187)
Figure 2 may be Figure 7 (Line 161 and 169)
Response 1: I checked the revised Word file and found that the cross-references to the figures in the paragraphs were all out of order across the board. It was corrected and converted directly to PDF, as noted by the reviewer.
Point 2: The title of the section 2 is Finite element model as same as that of section 3. it may be better to be another expression, for example, Simulation model. (line 60)
Response 2: In Chapter 2, an overview of the FEM model developed in this study was described, and in Chapter 3, the analysis results of the FEM model and the comparison with the actual accident were reviewed. However, the title of sections 2 and 3 is the same as the reviewer's opinion.
Therefore, the title of section 3 was changed to "Derailment simulation using simplified FEA model",
The title of section 3.1 was changed to "Derailment simulation method and setting variable factors". (line 142 and 143)
Point 3: In table 2, I think it is better to insert an empty line between data for EMU-260 and EMU-320 to improve readability. (line 89)
Response 3: Thanks for the feedback. I've added a column to the table to improve readability. (line 95)
Point 4: In my opinion, you should mention value of the friction coefficient μB used in this simulation as well as a reason why the value was chosen for this case. In the previous section, it was shown that coefficient μB has significant effect on phenomenon of the train after the derailment, so mentioning the choice of this parameter will make the results more convincing. (line 205)
Response 4: For the part of the simulation that is intended to replicate a real-world accident, I found that there are many conditions that did not mention, so it was included a more detailed description of the simulation. (line 206)
Point 5: In figure 10, an arrow to indicate distance between car no.4 and 5 is shown to be 200m. I think it is better to be shown as 50m. Correspondingly, it is better that the points at 150m from the derailment point and extra 50m are indicated by additional arrows in figure 11 for easier comparison to figure 10. It will give impression to the readers that the simulation result demonstrates good agreement with the actual accident. (line 215 and 217)
Response 5: In Figure 10, I noticed an unintentional misrepresentation of the length, so we corrected the original 200 m to 50 m, as suggested by the reviewer, and included a total distance of 200 m in the figure. (line 223)
Thanks for your comments and consideration.
Please see the attachment.
Regards.
Jaehoon Lim
Author Response File: 
Author Response.docx
Reviewer 2 Report
Dear Authors,
I have read the paper titled “Simplified Dynamic FEA Simulation for Post-Derailment Train-Behaviour Estimation through the Enhanced Input of Wheel-Ballast Friction Interactions” submitted for consideration in Applied Sciences.
This article was well-written and addressed a topic of interest to the scientific community worldwide. It described the development of a simplified finite element analysis model to predict train behavior after derailment. I appreciate the effort made to reproduce these events and to validate the model by comparing it to a real event. However, I felt that the results obtained by the model and presented in Figure 11 did not compare well with the real events presented in Figure 10. Because this was the only comparison presented between real events and the models created, it is unclear how these simulations would help address the question of what happens after a train derails and what effect the derailment would have in the areas surrounding the tracks. This lack of agreement also leads me to question the validity of the results presented in Appendix A. The last sentence of the abstract states that “various accident cases in Korea are compared and reviewed to verify the proposed model,” however, only one of these comparisons was presented. Since the agreement between that real event and the model was questionable, it is essential to present how well the models compared to these other accidents. As presented, I find it difficult to understand how these FEA models could be used to accurately predict the train behavior post-derailment.
Please find below specific comments on different lines of the text.
Sincerely,
Reviewer
General Comments:
It would be useful to refer to each figure and table in the text, presenting a short description of what the reader should expect to find in the plots, tables, or figures. Unfortunately, this was only done for some of the figures and tables.
Specific comments and suggestions:
Table 1, third column, fourth line: “1 locomotive and 4 cars were crushed or rolled over” instead of “1 locomotive and 4 cars was crushed or rolled over”
Line 49: “…field experiments have been conducted” instead of “…field experiments have conducted”
Line 81: error related to figure number
Table 2: it would be useful to include a larger space between the specifications (2nd column) of the EMU-260 and the EMU-320 trains
Line 125: error related to figure number
Line 132: error related to figure number
Line 156: error related to figure number
Line 161: “Figure” is listed twice and I believe the Figure number should be 7, not 2
Line 163: “speed difference between the leading and following cars” instead of “speed difference the leading and following cars”
Line 167: “Figure 7” instead of “Figure 2”
Line 173: “with a real-world accident” instead of “with an real-world accident”
Line 187: error related to figure number
Line 209: “in a real-world railway track” instead of “in an real-world railway track”
Figure 10: the labeling of 200m and 150m in the figure is misleading – it implies the drawing is not to scale and that a distance of 200m exists between cars 4 and 5, but I believe what the authors meant was that there are 200 m between car 4 and the derailment point.
Figure 11: what are the units of U? In addition to the comments made in lines 206 to 214, there seems to be other sources of discrepancy between the simulation results and the real accident. For example, the distance between car 8 and the derailment point seems a lot larger in the model than in the real-world scenario presented in Figure 10. The positioning of cars 6 and 7 also seem to differ. I would be interested in the authors discussion on these aspects as well.
Author Response
Response to Reviewer 2 Comments
I'm attaching a paper that has been revised for your comments, and I've written a response to your comments at the end of the file.
Please see the attachment files.
Point 1: It would be useful to refer to each figure and table in the text, presenting a short description of what the reader should expect to find in the plots, tables, or figures. Unfortunately, this was only done for some of the figures and tables.
Response 1: I checked the revised Word file and found that the cross-references to the figures in the paragraphs were all out of order across the board. It was corrected and converted directly to PDF, as noted by the reviewer.
Point 2: Table 1, third column, fourth line: “1 locomotive and 4 cars were crushed or rolled over” instead of “1 locomotive and 4 cars was crushed or rolled over”
with a real-world accident” instead of “with an real-world accident” (line 173 and 209)
Response 2: I've also made general grammar and wording fixes. (Table 1, line 173 and 209)
Point 3: Table 2: it would be useful to include a larger space between the specifications (2nd column) of the EMU-260 and the EMU-320 trains
Response 3: Thanks for the feedback. I've added a column to the table to improve readability. (line 95)
Point 4: error related to figure number (line 81, 125, 132, 156, 161, 167 and 187)
Response 4: As mentioned previously, I've fixed cross-references in figures and tables across the board.
Point 5: Figure 10: the labeling of 200m and 150m in the figure is misleading – it implies the drawing is not to scale and that a distance of 200m exists between cars 4 and 5, but I believe what the authors meant was that there are 200 m between car 4 and the derailment point.
Response 5: In Figure 10, I noticed an unintentional misrepresentation of the length, so we corrected the original 200 m to 50 m, as suggested by the reviewer, and included a total distance of 200 m in the figure. (line 223)
Point 6: Figure 11: what are the units of U? In addition to the comments made in lines 206 to 214, there seems to be other sources of discrepancy between the simulation results and the real accident. For example, the distance between car 8 and the derailment point seems a lot larger in the model than in the real-world scenario presented in Figure 10. The positioning of cars 6 and 7 also seem to differ. I would be interested in the authors discussion on these aspects as well.
Response 6: The unit for U in Figure 11 is a meter.
The reason why the displacement of the driving direction is larger than the actual accident is the fact that there are many simplified elements in this FEA model. In actual derailment accidents, the vehicle's wheels collide with rails, sleepers, fasteners, etc., and energy dissipation occurs due to car breakage and connections. Still, these factors are excluded in this paper. Therefore, as the reviewer commented, we have added and explained them. (line 214 to 225)
Thanks for your thoughtful comment.
Please see the attachment.
Regards.
Jaehoon Lim
Author Response File: Author Response.docx
Reviewer 3 Report
In this paper, the authors have presented an approach to evaluate the trains derailment through FEA. A simplified model (at the intermediate point between the simplest 2D and detailed 3D models) is proposed.
Authors have compared the numerical results with a real case.
The literature review is supporting the objectives, although it could be improved if additional FEA-review is added. The current literature misses FEA details, that has been simplified in the implementation. They should be discussed to show the real motivation for this simplified method (e.g. model complexity or computer load).
Regarding the model, some details are missed. More details should be included, in the belief that other researchers might want to replicate it.
The paper is well-organized, although Point 2 and Point 3 have the same title. It should be changed or merged. Currently, both points are messy. Methodology and results (Figure 6) should be at different point.
It is an interesting topic that could deserve the attention of the community one it is properly arranged and some details are fixed.
Some general points and comments:
· Each figure or table should appear in the text, as they are supporting the statements.
· Figure 1d, if numbers are showing the car number, include them in the description, or remove them.
· Line 49: “been” is missed.
· In the Train model, EMU-320 is firstly introduced, but Table 2 also describes EMU-260. Please clarify why both models are described, and only one is named within the text.
· Most references of figures show errors (Error! Reference source not found).
· Train model is not detailed. If other researchers try to reproduce the simulation, they could miss some data of the model (type of contacts, type of element, mesh size, time step, any implementation to improve the convergence, …). Please complete this information.
· The data in Table 2 is not referenced. Please indicate where this information is located.
· If cars have been considered as rigid, please specify it.
· Point 3 has the same title as Point 2. Please modify it. Separate clearly the introduction and references, and the results (e.g. Figure 6).
· Line 161. “Figure” is repeated.
· Figure 6. How has this data been calculated?
· Figure 6f. What vR= angular velocity mean? It is not clear in the graph.
· Line 172 / Line 183. Avoid the use of pronouns “we”, “our”, …
· Figure 8. If the numbers are describing the car number, please specify it.
· Please clarify how the lateral forces are limited. If cants are not used, how does the model consider this restriction? What is the objective of Figure 9? What is the limit for the contact point between the wheel and rail (Line 194).
· Line 200. Why the speed has been fixed at 117 km/h. Please justify.
· Figure 10 shows 200 m and 150 m. They are not at the same scale. Nevertheless, Figure 11 show only one scale. There are differences if both are shown in the same scale.
· Homogenize the size of titles, legend, …, in the Figures.
· Line 239. What is the effect of considering the friction by the rails, sleeper and other structures? Is there any reference? It should be validated with a detailed 3D simulation, just to justify the advantage of skip them. Then, if they have a negligible effect, the proposed methodology could be interesting, as a simpler calculation.
· From the above, the advantages and the results, when this approach is compared to other alternatives, should be shown. The number of elements, the computational time, and other parameters could help to compare the results and the resources. According to the accuracy, it could help to decide which approach is the best one.
·
Comments for author File: 
Comments.pdf
Author Response
Response to Reviewer 3 Comments
I'm attaching a paper that has been revised for your comments, and I've written a response to your comments at the end of the file.
Please see the attachment files.
Point 1: The literature review is supporting the objectives, although it could be improved if additional FEA-review is added. The current literature misses FEA details, that has been simplified in the implementation. They should be discussed to show the real motivation for this simplified method (e.g. model complexity or computer load).
Regarding the model, some details are missed. More details should be included, in the belief that other researchers might want to replicate it.
The paper is well-organized, although Point 2 and Point 3 have the same title. It should be changed or merged. Currently, both points are messy. Methodology and results (Figure 6) should be at different point.
It is an interesting topic that could deserve the attention of the community one it is properly arranged and some details are fixed.
Response 1: As per the reviewer's comment, it was found that the details of the FEA were missing and that points 2 and 3 had the same title, so we have corrected the titles and made changes to the tables and figures across the board. (line 142, 143)
Point 2:
· Most references of figures show errors (Error! Reference source not found).
- Line 161. “Figure” is repeated.
Response 2: I checked the revised Word file and found that the cross-references to the figures in the paragraphs were all out of order across the board. It was corrected and converted directly to PDF.
Point 3:
· Each figure or table should appear in the text, as they are supporting the statements.
- Figure 1d, if numbers are showing the car number, include them in the description, or remove them.
- Figure 8. If the numbers are describing the car number, please specify it.
- Homogenize the size of titles, legend, …, in the Figures.
Response 3: The previous representation of vehicle numbers in the illustration was an attempt to convey the overall vehicle behaviour and severity of the accident, but as the reviewer commented, the lack of explanation could lead to confusion, so we removed it. (Figure 1d and 8)
There were also made a general correction to equalise the size of the titles and legends within the figures. (Figure 6)
Point 4:
· Line 49: “been” is missed.
- Line 172 / Line 183. Avoid the use of pronouns “we”, “our”, …
Response 4: There were double-checked the overall grammar and corrected articles and expressions. (line 25 to 61, and 177 to 179)
Point 5:
· In the Train model, EMU-320 is firstly introduced, but Table 2 also describes EMU-260. Please clarify why both models are described, and only one is named within the text.
- The data in Table 2 is not referenced. Please indicate where this information is located.
- Train model is not detailed. If other researchers try to reproduce the simulation, they could miss some data of the model (type of contacts, type of element, mesh size, time step, any implementation to improve the convergence, …). Please complete this information.
- If cars have been considered as rigid, please specify it.
Response 5: The EMU-320 and EMU-260 use the same vehicle body, and due to differences in details such as equipment, differences in operating speeds and arrangements exist. Therefore, both vehicles are described in Table.2 for future use. However, as the description of the contents was insufficient, we have added additional descriptions and added citations to refer to the specifications. (line 76 to 86, and Table. 2)
Point 6:
· Point 3 has the same title as Point 2. Please modify it. Separate clearly the introduction and references, and the results (e.g. Figure 6).
Response 6: In Chapter 2, an overview of the FEM model developed in this study was described, and in Chapter 3, the analysis results of the FEM model and the comparison with the actual accident were reviewed. However, the title of sections 2 and 3 is the same as the reviewer's opinion.
Therefore, the title of section 3 was changed to "Derailment simulation using simplified FEA model",
The title of section 3.1 was changed to "Derailment simulation method and setting variable factors". (line 142 and 143)
Point 7:
· Line 200. Why the speed has been fixed at 117 km/h. Please justify.
Response 7: The actual accident report confirms that the recorded speed at the time of the derailment was 117 km/h. However, we found that there was a lack of explanation, so we added it. (line 206 to 213)
Point 8:
· Figure 6. How has this data been calculated?
- Figure 6f. What vR= angular velocity mean? It is not clear in the graph.
Response 8: Figures 6a-e are graphs of the degree of deceleration of the first vehicle at the time of the derailment, holding the speed and the angular velocity of the first vehicle fixed and the friction conditions of the ground and the car. (line 159 to 166)
Figure 6f is a graph showing the first vehicle's deceleration as the first vehicle's angular velocity varies when the speed at the time of derailment and the friction conditions between the ground and the car are fixed.
However, this explanation was lacking in the previous paper, and cross-reference errors between the text and the figure occurred, so we have corrected that part. Also, the title of the figure has been corrected. (line 159 to 165, and figure 168)
Point 9:
· Please clarify how the lateral forces are limited. If cants are not used, how does the model consider this restriction? What is the objective of Figure 9?
What is the limit for the contact point between the wheel and rail (Line 194).
Response 2: This simulation model is basically applicable in a flat environment, and 'Cant' and 'Slak' are not considered. However, we added them because there was not enough explanation for them.
Also, the description of the transverse acceleration limit point was not sufficiently explained, so it was added. (line 206 to 225)
Point 10:
· Figure 10 shows 200 m and 150 m. They are not at the same scale. Nevertheless, Figure 11 show only one scale. There are differences if both are shown in the same scale.
Response 10: In Figure 10, we noticed that the length was unintentionally misrepresented, so we corrected 200 m to 50 m and included a total distance of 200 m in the figure. (Figure. 10)
Point 11:
· Line 239. What is the effect of considering the friction by the rails, sleeper and other structures? Is there any reference? It should be validated with a detailed 3D simulation, just to justify the advantage of skip them. Then, if they have a negligible effect, the proposed methodology could be interesting, as a simpler calculation.
Response 11: Currently, there are no formalised or standardised studies of structural friction caused by rails, sleepers and other structures, and the criticality of the effects is not clearly presented.
However, the experiments and 3D simulations referred to in the introduction briefly mention energy dissipation.
Therefore, there were added a reference to it. (line 220)
In addition, we are currently conducting long-term experiments and 3-D simulations to address these issues in a future paper. (line 252 to 255)
Point 12:
· From the above, the advantages and the results, when this approach is compared to other alternatives, should be shown. The number of elements, the computational time, and other parameters could help to compare the results and the resources. According to the accuracy, it could help to decide which approach is the best one.
Response 12: As mentioned in the introduction, this study, unlike the previous studies on train running stability and simulation of derailment situations, was conducted to check the post-derailment behaviour, and it is difficult to provide direct figures on the differences in the programs used, the number of elements, and the analysis time.(citation 15 to 18)
In addition, most of the 3-D simulation papers do not specify the simulation time or only check the behaviour immediately after the derailment, so comparison is not possible.
Thanks for your thoughtful comment.
Regards.
Jaehoon Lim
Author Response File: Author Response.docx
Round 2
Reviewer 3 Report
All open points have been solved and justified in this new version. From my point of view, this paper can be published.
