## 1 Introduction  
The future of the rail transport sector depends on reliable mobile broadband connectivity, powered by innovative fifth generation (5G) network technologies for more ecological, safer, and faster travel. The railway industry has adopted advanced 5G technologies to improve overall service operation by improving efficiency and passenger experience, generating higher train speeds by enabling autonomous trains, and increasing passengers and staff security by making the systems more intelligent [1, 2]. These connectivity improvements enable critical railway applications for automatic train control and protection, train localization, onboard system monitoring, and trackside equipment maintenance. The increased security provides access to real-time monitoring to ensure passenger safety and to protect assets. The advances in automation and data analytics for increased safety and efficiency stimulate the introduction of new applications. Critical railway operations are vital to system operation, and by adopting 3 GPP standardization for mission critical communications, both railways and passengers may benefit from standardized building blocks and devices. The Third Generation Partnership Project (3 GPP) standards for mission critical communications define the architecture and open interfaces that enable a high level of interoperability and simplify interconnectivity with future broadband networks [3, 4]. Mission critical voice, data, messaging, and video services are key elements for implementing the vision that rail systems have for their future mobile communication systems. Railway communications require high network availability, guaranteed quality of service for priority traffic, improved passenger safety through always-on communications, and reduced operating and maintenance costs. Technologies that support 5G connectivity and design principles such as network virtualization, network slicing, network soft-warization, and mobile edge computing can address the specific requirements of railway networks [5, 6, 7, 8]. The Future Railway Mobile Communication System (FRMCS), standardized by the European Telecommunications Standards Institute (ETSI) and the International Union of Railways (UIC), forms the basis of next-generation intelligent railways. It provides the means to improve rail operations and to offer innovative services to users. FRMCS standards define user requirements based on analyzing typical railway use cases [9]. Each use case is defined as a list of actions describing the interactions between the users and the system to achieve a goal [10]. The use cases related to critical voice communications include a feature called Role Management and Presence as a part of the user authentication and authorization process. The FRMCS standards also define the system architecture by analyzing the architectural requirements and the requirements for identification and addressing system security, and positioning [11]. The FRMCS system architecture creates clear separation between the application stratum, service stratum, and underlaying transport stratum. The research presented in this paper is positioned in the service stratum. The FRMCS standards do not postulate how the identified railway requirements might be implemented. Future FRMCS standardization activities will include defining the building blocks and interfaces, provisioning communication services to the application layer, and ensuring interoperability. In this paper, a service-oriented approach for the implementation of the Role Management and Presence feature is presented. Two services for railway identity management are depicted for use as service bricks in order to realize FRMCS security mechanisms. Existing works focus on problems related to railway security risks [12, 13]. Exposure to potential malevolence can cause substantial impacts on human safety and infrastructure.