## 3 The Overall Ontological CBR Framework for Metro Accident Responses
### Knowledge Representation in the Ontology Model
#### 3.2.3. Accident Cases Formalization  
The aforementioned ontological model provides a unified semantic structure for the standardized expression of metro accident cases. Based on the semantic framework above, historical accident cases can be represented in a standard semantic format to facilitate computer analysis and processing. Based on the developed ontology model, the metro accidents can be described by a set of attributes that can be divided into three main categories, namely, accident characteristics, accident response, and post-accident assessment. The name of these attributes is defined in the classes and properties of the ontology model, while the value of these attributes is based on the individuals of the ontology model. The details of the main attributes are shown in Table 2.  
Figure 3: The top-level ontological model for metro accidents. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Category** & **Attribute** & **Type** & **Meaning** \\ \hline \multirow{6}{*}{Acident characteristic} & Operational line & Integer & The metro operation line where the accident happened \\  & Station & String & The metro station where the accident happened \\  & Section & String & The metro station where the accident happened \\  & Train number & String & The code of the train that had the accident \\  & Fault characteristic & String & The representation of the train’s abnormal operating state caused by the accident \\ \hline \multirow{3}{*}{Accident response} & Response staff & String & The staff member who takes the accident response measures for the accident \\  & Response measure & String & The response measure taken by the accident staff for troubleshooting \\  & Response resource & String & The response equipment and supply for assisting the implementation of response measures \\ \hline \multirow{6}{*}{Post-accident assessment} & Death number & Integer & The number of deaths caused by the accident \\  & Injury number & Integer & The number of injuries caused by the accident \\  & Economic loss & Float & The economic loss caused by the accident \\  & Accident impact & Delay time & Date & The delay time of the train caused by the accident \\  & Delayed trains & Integer & The number of delayed trains caused by the accident \\  & Lost trains & Integer & The number of lost trains caused by the accident \\  & Empty trains & Integer & The number of empty trains caused by the accident \\  & Accident level & String & The severity of the accident \\  & Accident cause & Accident type & String & The accident type according to the accident cause \\  & Fault cause & String & The main reason for the accident \\  & Corrective measure & Corrective measure & String & The corrective measure according to the accident cause \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The details of the main accident attributes. Therefore, a case can be expressed as a three-tuple, \(C=(C_{c},C_{r},C_{p})\), where \(C_{c},C_{r}\), and \(C_{p}\) refer to a set of attributes that describe the accident, its accident response, and its post-accident assessment, respectively. For a given case base \(C_{b}\), \(C_{i}\) is the sequence number of a case in \(C_{b}\) (\(1\leq i\leq n\), where \(n\) is the total number of cases in \(C_{b}\)). \(C_{ic}\), \(C_{ir}\), and \(C_{ip}\) represent the accident characteristic, accident response, and post-accident assessment of case \(C_{i}\), respectively. Web Ontology Language (OWL) was selected to encode the metro accident knowledge. OWL enables the knowledge to be linked together and represented semantically in a semantic network. The accident knowledge is exported into an \"OWL\" file that can be managed by the ontology management tool. Based on the concepts in the developed ontology model, textual accidents can be represented in a standard format, which can be used for further retrieval. In addition to the case retrieval, the corresponding responses to the accidents should also be obtained.