## 6 Conclusions  
Prior case studies related to the maintenance of railway vehicle products are insufficient. In this study, AR-based training content for railway vehicle maintenance was developed to overcome the drawbacks of booklet-type manuals and improve the proficiency of maintenance operators. Additionally, AR technology was applied to the maintenance procedure of the axle-mounted disc brake system, which is an important component of railway vehicles. Feedback collection was conducted in two ways: a SUS survey and an evaluation of training efficiency. The conclusions obtained from these efforts are as follows:  
1. The axle-mounted disc brake system maintenance manual was loaded on the mobile devices, reflecting the requirements of railway vehicle maintenance operators and real situations of working sites. Therefore, maintenance operators can proceed with individual training at a working site without using an existing booklet-type manual.
2. To reduce human error, dangerous/difficult maintenance procedures are presented in a way that raises the awareness of operators. Visualizing the knowledge of veteran maintenance operators makes it easier to transfer experience.
3. To improve training efficiency compared to that of existing booklet-type manuals, the checklist and top view screen are displayed. Transparency control is applied to the product to display the connections within the product hierarchy.
4. The measurement operation and functional check operation are visualized to reduce the uncertainty of the dimension measurement and to provide indirect experience of the working environment. As a result, maintenance operators can experience the maintenance manual in AR, leading to improved maintenance proficiency.
5. A SUS survey was conducted to evaluate the usability of the AR content. The mean score was 73.36, and a grade of B (Excellent) was obtained, which means that this content is suitable for use in training for railway vehicle maintenance.
6. A test with 20 questions was conducted with two groups to evaluate training efficiency. As a result, the mean score of the group using AR content was 16.2 points higher and showed a 34% improvement in training efficiency.  
In future studies, it will be necessary to develop AR content for brake operating units and air compressors and maximize the training efficiency of railway vehicle maintenance.  
Conceptualization, [PERSON]-H.P. and C.-S.K.; validation, H.-J.K. and S.-I.L.; writing--original draft preparation, H.-J.K., S.-I.L. and C.-S.K.; writing--review and editing, H.-J.K. and C.-S.K.; visualization, [PERSON]-H.P. and C.-S.K.; project administration, C.-S.K. All authors have read and agreed to the published version of the manuscript.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Features** & **Actions** \\ \hline Reducing human error & (a) Warning of dangerous and difficult maintenance procedures \\  & (b) Sharing knowledge of existing veteran maintenance operators \\ Improving training efficiency & (c) Offering visualized checklist \\  & (d) Indicating connectivity within product hierarchy \\ Virtual experience based on manual & (e) Providing virtual experience of measuring dimensions \\  & (f) Providing virtual experience of functional check operations \\ \hline \hline \end{tabular}
\end{table}
Table 7: Key features of the AR content.  
**Funding:** This research is supported by the Korea Agency for Infrastructure Technology Advancement (KAIA) grant funded by the Ministry of Land, Infrastructure and Transport (Grant21 QPWO-B152223-03).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.