# Supporting the Management of Rolling Stock Maintenance with an Ontology-Based Virtual Depot
## 4 Development of the Abstract Ontology  
To inform the development of the ontology for the Virtual Depot created in this study, published ontological knowledge was reviewed to help build up the underlying conceptual model. Two schemas that were used to inform the high-level rolling stock ontology included in the 'Virtual Depot' tool were:  
Figure 1: Four-phase approach to virtual depot building. * The RailML [19] standard, which provides a set of XML schemas to enable interoperable communication between heterogeneous railway applications. The rolling stock schema includes a high-level description of rolling stock-related concepts. * The RSSB Cross Industry RCM project [20] defined a rolling stock XML schema, including some structural aspects such as parts of the vehicle, for example, bogies (a bogie is a structural element of a train which contains two wheelsets and associated traction and braking equipment and suspension components) [21; 22]. Despite the relevance of this schema, they are limited and need to be extended and formally captured in order to be of use within the Virtual Depot. Two other related ontologies were used to help populate components within the Virtual Depot:  
* The Smart Rail [23] ontology includes both the description of high-level-depot and rolling-stock-related concepts, and hence, it is very useful for our purposes. This ontology includes a location definition that we use to record the location of the train and also the depot locations. The measurement units related to the remote condition monitoring/inspection data are also included. To capture measurement and measurement units [24], we utilise an existing ontology (QUDV [24]. * The Vehicle ontology [25] is not related to railway vehicles; however, it makes use of two other ontologies, time and space location, which are needed in our context. Armed with these pre-existing works, the Virtual Depot's central ontology was designed using experts' knowledge and maintenance manuals in addition to an existing rolling stock virtualisation proposed by [PERSON] et al. [26]. We used OWL ([[https://www.w3.org/OWL/](https://www.w3.org/OWL/)]([https://www.w3.org/OWL/](https://www.w3.org/OWL/))) as the language of capture, on account of it being a standard. As well as it is being well-known and well-studied, an ontology written in OWL has the benefit of a wide range of tools available to manipulate it, reason with it, and translate it to other formal languages. While OWL is not as expressive as full first-order logic, it is appropriate for the knowledge we capture where we require only light reasoning capabilities, such as inheritance, inferences, and consistency checking. Additionally, we have shown how to extend the ontology with rule knowledge, where required, by the use of SWRL [26]. To accurately capture the complex structure of a railway maintenance depot, our ontology describes both tangible (physical) and intangible (information) entities, such as vehicles or roads and maintenance activities or plans, respectively. This dual representation is necessary for realising a maintenance depot and facilitating its operation. A multitude of semantic relations are included to model the maintenance environment including typology (for instance, types of resources), membership (such as listing the parts of a vehicle), and crucial semantic associations (as an example, required entities for realising a maintenance activity). In essence, the design choices of classes and semantic relations combined with the reasoning capabilities of OWL are structured to address a series of competency questions, indicatively: (a) What types of maintenance activities are scheduled for a specific type of vehicle? (b) Which resources are required for a particular maintenance activity? (c) What is the availability of a particular maintenance road or shed for a scheduled maintenance task? (d) How often does a specific vehicle component require maintenance? (e) Can a maintenance activity be performed given the current availability of resources and staff?