## 4 Technology Development Strategy of TFIFT Systems
### Results  
The technological development of the TFTFT system was confirmed by applying various methods. First, a GTM-based patent map was created to detect patent vacuums. As a result, a total of 10 patent vacuums were found, and 6 patent vacuums were confirmed, excluding empty vacuums. Second, we conducted a technical analysis to confirm the promise of the patent vacuums. A technical analysis consisting of a criticality analysis to confirm technology level using a technical level map considering the quantitative and qualitative perspectives of the technical field and a trend analysis to confirm future promise using a time series analysis were conducted. The criticality analysis classified the technical level into high, medium, and low levels, and the trend analysis was categorized into hot, active, and cold fields. Comprehensively, we have identified patent vacuums in the high level-hot field area. Third, an IPC network was used to analyze the macroscopic aspects, and the keyword network was used to analyze the relationship between the technical elements in order to derive a promising patent vacuum technology development strategy. In the technical analysis, it was found that the development of the 9 th and 10 th patent vacuums was the priority among patent vacuums. When planning vacant technology development, patent vacuums must be considered promising technology fields. Vacant technology is a technology that has not yet been developed, making empirical decision-making difficult. In order to design a concrete technology development strategy, we conducted the network analysis using IPC codes and technology keywords. As a result, the 9 th patent vacuum contains the IPC codes of B60P and B61, which are technical field of vehicles suitable for freight transportation and the technical field concerning railway vehicles, so it is defined as the technical field of rail vehicles suitable for TFTFT systems. E04B, E04H, B60F, B62D, B61F, B61L, B65D, and G06Q were derived via the IPC network as technical fields that need to be investigated in advance for the purpose of developing the 9 th patent vacuum. These IPCs consist of the structural and vehicle design suitable  
Figure 8: Results of the keyword network analysis: (**a**) 9 th patent vacuum and (**b**) 10 th patent vacuum. for TFTFT system's flatcars or trains. As a result of applying the keyword network, \"control,\" \"lock,\" \"load,\" and \"assemble\" were confirmed as the significant technical elements. Transportation vehicles need to be coupled with various means of transportation, rendering locking essential. The 10 th patent vacuum was confirmed by the core IPCs for B65G and B61D. B65G is the technical field of equipment for container freight transportation and storage, and B61D is the technical field of railroad vehicles. Therefore, the 10 th patent vacuum can be defined as the technical field of equipment and systems for container freight transfer to rail vehicles. We confirmed that the technical fields that need to be investigated in advance to develop the 10 th patent vacuums are B61B, B61L, B63B, B66C, and E01B via the IPC network. Since these IPCs contain technology for railroad machinery and container freight loading/unloading equipment, they should be investigated and analyzed for technological development concerning efficient container freight transportation. Furthermore, as a result of applying the keyword network, \"frame,\" \"vibrat,\" and \"signal\" were confirmed as the significant technical elements. Container freight includes not only general but also liquid/gaseous special freight, so this must be taken into consideration. Unlike general freight, special freight requires a different container structure, which means it is necessary to pay attention to minute vibrations during transportation. This study found two technical fields for developing a vacant technology for the piggyback flatcar transportation system and derived the IPC fields and significant technologies that need to be investigated in advance for their development.