Analysis of Maintenance Techniques for a Three-Dimensional Digital Twin-Based Railway Facility with Tunnels
Round 1
Reviewer 1 Report
Dear Authors,
Thank You for so interesting and relevant Article entitled "Analysis of 3D digital twin-based railway facility (tunnel) 2 maintenance techniques".
1. The article is written on a relevant topic, well-structured, and logically proven.
2. The topic is highly original and relevant in the field.
3. The authors duly describe the subject area compared with other published material.
4. The authors could consider some minor improvements in the logic of the presentation of the results:
- adding in the end of the Introduction section the research gap, research aim and the scientific authors' contribution;
- considering in the end of the Results some paragraphs about the possibility of finding a stable solution;
- describing the topic for future research in a separate section 4. Discussion. I'd recommend adding to the discussion section some sentences describing the debate in the context of broad implementation in various fields of economic activities.
5. The conclusions are consistent with the evidence and arguments presented. I’d recommend the authors should include in Conclusion by choosing one paragraph from each section and the issues for future research.
6. The references are appropriate.
7. Please disclose the Results in lines 194-223 in the logic connection with the research aim added in the Introduction.
I hope my recommendation could help improving the manuscript. After these minor improvements I'd recommend the acceptance of this Article,
Author Response
Please see the attachment
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The paper claims, in the abstract, to have conducted the first domestic applicaton of 4th industrial technologies to a railway tunnel.
However, there is already a plethora of system providers and frameworks on the global market. This is completely lacking in the introduction and the literature references.
Something like https://www.mdpi.com/2076-3417/12/22/11310/html or https://www.sciencedirect.com/science/article/abs/pii/S0924271618302028. Please add the references that appear most relevant to your solution, and add information how your solution and approaches differ.
The second claim in the Abstract again is questionable, namely that a 3D digital twin ... could not be obtained with other fixed LiDAR systems. This would require a thorough study applying a critical mass of such systems, with quantitative and qualitative comparisons.
The introduction gives a profound overview on the objectives and programmatic background of the presented study, also with some international context.
It mentions a 13-km long test track, which could be highly interesting also for the international surveying community: It would be valuable to have some further documentation or related studies added to the references. Already a quick www survey shows several such links.
At the end of the introduction, a short outline of the paper would help, to follow the rationale of the paper and point out the context of its claims.
The description of the test site restricts to the length and summary of covered lenghts, but does not refer to additional quantitative and qualitative specifics such as tunnel shape / diameter, variety of installations, or quality & maintenance condition.
The test site overview is followed by a sparse description of the selected scan system, capture software, and virtual 3D model assembly. Sensor orientation by means of repeatedly applied QR codes seems straightforward, yet the shape of the ground plan (as depicted in Figure 7b) is not clear - is this derived from attaching adjacent scans to each other up to the claimed lengths of several 100m as mentioned in Section 2.1? This would severely suffer from drifting artefacts, such are not reported, and the examples on Figure 5 and 6 seem to show only small parts, not entire longer segments.
The 3D Digital twin program section is very sparse: The resulting space details, schematic floor plans and BIM would be interesting to show. Were such generated from the entire claimed length of 2.265km? What is the processing effort / cost / manual check & edit effort per km? Were such / all results presented to the entities / customers mentioned in the introduction, and if so what was their assessment & use?
The association between Figure 7 and its referring text (this approach can be used...) is unclear: What is the message here?
This section on digital twin is followed by an attempt of comparative LiDAR techniques' analysis. The presented table, however, seems to compare only the data sheets, and not a real analysis based on a representative case study in the mentioned test environment. Please confirm, or add quantitative analysis results, such as sigmas, coverage percentages, drift errors, and also qualitative features such as data interfacing, acquisition complexity (e.g. power supply, robustness to tunnel conditions, illumination effects, etc.), cost, acquisition duration, the stability of the control and interfacing software and further implications that would bring added value to the reader.
In Section 3.3 the claim "identical to the real space" shall be put into proper perspective.
The title is misleading. The term "digital twin" in the paper seems to be understood in the sense of just the shape, i.e. point clouds, and not in the general sense, as to have a real 3D, textural, and semantic model of the inspected edifice. Whilst this, to some extent, is mentioned (without details) in Section 2.2.3.3., the latter does not apply at all to the brief comparative analysis in Section 3.2.
The title should therefore be modified to better focus on the 3D shape and point clouds as mainly discussed in the paper.
The Conclusion mentions some issues, prospects and aspects in most part already known by the international tunnel survey, documentation and maintenance community.
Further it claims a contribution to the establishment of a national policy: It is recommended to first seek out for existing frameworks that have been developed for decades in the tunnel maintenance domain, the study described here seems to cover only a small glimpse of all aspects that would be required for such a high-level claim.
Some sentences are unclear to the reviewer. What does "a single 3D space platform is produced through a matching process" mean?
Figure 4: please make sure you have consent to publish the picture from the person depicted.
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Thank you for elaborating a revised version.
Some of the improvement potential was met, thank your for those.
I refer to remaining issues in the following.
2. the citations provided by the 1st review were only examples and should guide the Reader to further relevant literature about existing solutions. If the Authors find them improper (which may be the case), please find a set of others that give the reader a profound basis to understand the novelty of your work. See also Comment 14: "seek out for existing frameworks that have been developed for decades in the tunnel maintenance domain"
5. the suggestion to present an outline of the paper at the end of the introduction was not followed up. Take a look at current literature, this is state of the art. e.g. something like "The remainder of this article is structured as follows. ..."
7. You explain something in your response which did not find its way into the revised paper. Further, the explanation in the cover letter does not answer the question how the QR codes were used to model the tunnel ground (floor) plan, where that came from, and how adjacent scans were attached and finally geo-referenced, and how drifting was tackled.
8. still no floor plans or BIM is shown. L145 sentence is incomplete. What means "real-time monitoring" in this context? Somebody with tape measurements taking reference distances etc.?
10. still no quantitative results are present. L205-207 talks about "can be assessed" (did you? Which results compared to which reference?), and the mentioned L284-287 are already references. If no quantitative analysis has been made, please state this in the paper and justify.
11. "identical" is still present in L135. I don't see the new term in Lines 216/217 in my version platforms-2142527-peer-review-v2.pdf
16. Line 297 in my version is already in the references' section
Parts of the new text should be revised in style and message. They suffer from very complicated or unclear language which should be made more concise to read properly. It is suggested to have at least these portions reviewed internally by an experienced English writer.
Some examples are
- L79 & 80 repetition of "In this study"
- L45 repetition of "3d modeling ..."
- L47ff multiple repetitions of citations
- L49 "describes the contents of examining..."
- L51 same issue repeated
- L81 twice "developed"
- L155 "Figure 7 is a unique method...." should read "Figure 7 shows a unique method...".
- L156 "a monitoring technique that can be reviewed by converging..." - what do you really want to say? "reviewed" and "converging" in this context sound very strange.
- L166 Figure 6 caption "Railway comprehensive test track tunnel according to the 3D digital twin model" - what do you really want to say? "Examples of resulting textured point clouds, attached to each other, in 3D viewer provided by Matterport"?
- L167 Table 1 caption "Physical structure of tunnel among railroad comprehensive test tracks" - what do you really want to say? "Dimensions, shapes and conditions of test tracks"?
- L196 "SLAM-based lidar scan is implemented as a 3D point cloud, which is an aggregation of point clouds" - "Implemented" sounds very strange here. And a 3D point cloud is an aggregation of point clouds??
- L270/271 "based on" repeated
- and many more
Table 1 I guess you mean "NATM" instead of "MATM"
L133 "After scan data collected via Matterport Capture software have been uploaded" - not a sentence. Uploaded to where?
Please use proper descriptions in figure captions. E.g. it seems that Fig 7a shows a tunnel profile as output of a VR workflow, therefore the caption may be something like "Appearance of the resulting textured point cloud in the Matterport VR Workflow"
Author Response
"Please see the attachment."
Author Response File: 
Author Response.docx
