Figure 4
The diagram is divided into four quadrants by dashed lines labeled Niche Themes, Motor Themes, Basic Themes and Emerging or Declining Themes. The x-axis represents the degree of relevance or centrality, and the y-axis represents the degree of development or density. The upper-right quadrant contains two clusters: digital twin, digitalization, and machine learning; and sustainable development, logistics, and freight transportation. The upper-left quadrant includes cyber security, cybersecurity, and intelligent systems. The lower-left quadrant has industry 4.0, internet-of-things, and investments. The lower-right quadrant features railroads, railroad transportation, and rails. Another cluster in the lower-right quadrant includes digital transformation, information management, and sustainability. The position of clusters as motor themes indicates the structural importance and interconnectedness of these keywords within the research network.Thematic map. Source: Authors’ own work