Railway Safety according TSI

RAMS in railway applications: 
legislation, standards, and methods

Ensuring reliability, availability, maintainability, and safety (RAMS) is a crucial aspect of the development and operation of railway systems. With the political goal of interoperability of European railways, the technical requirements for the railway systems, the processes to be followed and the methods to be used have been standardised. In addition to meeting technical requirements, a railway system that complies with the requirements – an approvable railway system also requires a multi-layered process, which is defined in parts, comparable to the technical requirements, in Technical Specifications for Interoperability (TSI) and the  details of which are laid down in "harmonised" EN standards. TSIs and harmonised EN standards thus form the legal and technical framework for the entire life cycle of a railway system and the basis for its safe, reliable, and interoperable operation.

Interoperability Directive and TSIs as a legal framework for rail systems

The legal framework for railway systems in the European Union is based on the Interoperability Directive and the Technical Specifications for Interoperability (TSIs). Compliance with these provisions, or the fulfilment of national legislation in the case of a directive, are binding on the respective systems concerned.

The most important TSIs include:

1. Interoperability Directive (2016/797): Defines conditions for interoperability within the European Union's railway system and defines its subsystems, from a structural and functional point of view.
2. Infrastructure TSI (1299/2014): Regulates the infrastructure subsystem and ensures compliance with interoperability requirements for new and modernized rail infrastructure.
3. TSI Energy (1301/2014): Regulates the energy subsystem and sets standards for power supply, energy efficiency and maintenance.
4. TSI Rolling Stock (1302/2014): Defines conditions for the interoperability of locomotives and rolling stock for passenger transport to ensure that they meet both safety and performance criteria.
5. TSI Control-Command and Signaling (2023/1695): Defines conditions for control-command and signaling, including automated train operation, and improved digital communication. Replaces Regulation (2016/919).

EN standards and the V-cycle in RAMS

The EN 50126 series, with the V-cycle defined therein, forms the foundation of the RAMS process in railway applications in the European Union. This railway-specific V-cycle enforces a systematic approach by defining in detail both the content and the verification and validation activities for each lifecycle phase, thus ensuring the reliability and safety of the system throughout its lifecycle.

The EN 50126 series is completed by EN 50657, which defines the life cycle of on-board software. 

For the highest safety requirements of train control, train command and signaling (CCS) systems, the life cycles are defined in EN 50128 for software and EN 50129 for electronics. 

Together, these standards provide a comprehensive framework for achieving consistent RAMS targets across all subsystems of a rail project.

RAMS Implementation Methods

The main methods referred to in the EN standards include:

• Failure Mode and Effects Analysis (FMEA): Identifies potential failures within a system and helps mitigate risks early in the design phase.
• Fault Tree Analysis (FTA): Analysis to determine the probability of occurrence of a particular fault at the observation level by analysing the probability of occurrence and the interaction of causal or contributing occurrences.
• Reliability Block Diagrams (RBD): Methods for modelling and for the qualitative and quantitative determination of system reliability.
• Determination of the Safety Integrity Level (SIL): Qualitative allocation and determination 
• Hazard log management: Throughout the lifecycle, a hazard log tracks identified risks and the implementation of risk mitigation measures, ensuring that all hazards are systematically managed before the system is deployed.

These methods ensure that the railway systems meet both the technical and safety requirements set out in the relevant standards and take into account risks across hardware and software systems.

Conclusion: The importance of compliance

In railway projects, compliance with both EN standards and TSI is not only a technical necessity, but also a legal requirement. Standards such as EN 50159 ensure that critical communication systems maintain their integrity even in challenging environments, while the more comprehensive standards such as EN 50126 ensure that all aspects of rail operations meet the stringent RAMS targets. 

As railway systems continue to evolve, it will be crucial to keep up with both legislative changes and technical standards. Future articles will look at specific aspects of RAMS, such as identifying hazards and developing safety evidence, and provide deeper insights into the methods that support safe and reliable rail operations.