Exploring LoS in Railway Transportation Systems using SysML

Railway Transportation Systems (RTSs) are systems of high complexity consisting of other systems. Thus, they may be studied as Systems-of-Systems (SoS) using the Systems Modeling Language (SysML). Since their purpose is to provide transportation services that have a critical role in urban societies and affect millions of users, they have to be precisely evaluated, focusing on their adequacy to deliver the expected service quality. In the case of RTSs, service quality is expressed by Level of Service (LoS) indicators related to the passenger's experience within RTS components, such as stations and trains. Such indicators express, for example, space and time restrictions, and are standardized in international bodies. In this paper, an approach, integrating LoS concepts in Model-Based Systems Engineering (MBSE), is proposed, enabling the description and verification of LoS requirements for RTS SysML models. Exploiting SysML, a RTS model can be generated, populated with basic transportation entities and specific LoS requirements associated to them. Requirements are described in a simple, abstract fashion by the RTS engineer, while more specific requirements are automatically generated and associated with basic RTS components. Furthermore, verification of RTS LoS is facilitated, helping the system designer to assess alternative RTS operation scenarios. In this context, a case study discussing the Athens Metro System is briefly examined.