## 2 Materials and Methods  
We analyse journal references, practical studies and commercial systems pertaining to RTSSI-relatable visualisation frameworks. For this purpose, we partition notion of an RTSSI visualisation framework into two sequential phases: (1) Data Acquisition Approaches; (2) Information Conveyance Schemes. A subtle remark, note that information is not itself raw data but the meaning from raw data. The tree holds 5 apples' is raw data, but knowing we expect it to hold 20 gives meaning to the data (i.e., the harvest is poor). Context and analysis turn raw data into useful information.  
No prior review work encountered considers both described phases in detail. In fact, we found only two literary reviews directly related to RTSSI [10,11]. Both principally analyse literature concerning phase (1), of which [10] is the only dedicated review article. However, being published in 2015, it now lacks current relevance due to technological advancements. Discussions of ROBO-SPECT (RS) are the only RTSSI-specific scheme we do not consider outdated, but consideration will only be paid its most recent publications. The 2015 review provides a comparative 'baseline' for discerning small updates on already established methods from genuinely novel RTSSI innovations. Our review focuses on the latter as this provides greater benefit to current researchers and practitioners, with [10] providing historic reference. By contrast, we note although [11] is the more recent publication, it relies on many references previously provided in [10]. Our observations clearly necessitate creation of a novel review addressing both phases of RTSSI visualisation framework development, spanning research projects and commercial systems documented between 2015 and 2021.  
Our review procedure is illustrated in Figure 1 and draws upon knowledge from over 300 information resources. We formally cite 255 primary resources (e.g., reviews, articles, proceedings, etc.). Upwards of 60 secondary resources (e.g., internal reports, cooperate multimedia, web-resources, etc.) provide further insight, however are not formally referenceable Owing to the fundamental dependency phase (2) has on phase (1), materials considered have frequently contributed to discussion of both areas. As a conservative estimate, the distribution of total literature is approximately 67% across phase (1) and 55% across phase (2), with 33% providing contextualisation. Maximal crossover is approximately 69%. Academic publications are obtained by 360-degree searches of journal articles and conference proceedings through databases including IEEE Xplore, Springer, MDPI and Google Scholar. Materials concerning commercial technology solutions are primarily sourced from the relevant corporate organisations' website, system manuals and any associated technical papers. Keywords recurringly searched include: \"Railway Tunnels\", \"Structural Health Monitoring\", \"Subsurface\", \"Defects\", \"Computer Vision\", \"Visualisation\", \"Human Computer Interaction\", \"LiDAR\", \"Photogrammetry\", \"Ground Penetrating Radar\" and \"Extended Reality\". For a detailed breakdown of key references, see Appendix A.  
Figure 1: Flowchart of reviewing procedure prescribes handling, analysis and management of resources, alongside providing a rigorous procedure for appraising the effectiveness and scope of frameworks considered.