EN 61375-2-5: Powering Europe’s Digital Trains

What is EN 61375-2-5: Ethernet Train Backbone (ETB)?

EN 61375-2-5 is a European standard that specifies the requirements for the Ethernet Train Backbone (ETB). The ETB serves as a high-bandwidth communication network that runs along the entire length of a train, connecting different vehicles or consists. It is a critical component of the modern Train Communication Network (TCN), designed to support data-intensive applications such as passenger information systems (PIS), video surveillance (CCTV), and on-board internet access.

The standard defines the communication protocols, network architecture, and physical layer requirements to ensure reliable and interoperable data exchange between train cars. Unlike older, lower-bandwidth train buses, the ETB leverages standard Ethernet technology, adapted for the demanding railway environment, to provide the capacity needed for modern digital services on rolling stock.

Key Technical Principles of the Ethernet Train Backbone

The EN 61375-2-5 standard is built upon several core technical principles to ensure its functionality, reliability, and interoperability in a dynamic train environment.

Network Architecture and Topology

The ETB employs a specific network topology to handle the linear and variable nature of a train. Key characteristics include:

• Linear Bus Topology: At its core, the ETB is a linear network that extends from one end of the train to the other. Each car or consist contains ETB nodes (typically managed Ethernet switches) that are interconnected.
• Redundant Links: To ensure high availability, the standard mandates a redundant physical connection between adjacent vehicles. This is typically implemented as two separate Ethernet links, creating a chain of interconnected rings or a redundant line topology.
• Automatic Configuration: The network is designed to be self-configuring. When train cars are coupled or decoupled, the network must automatically detect the new topology and establish communication paths without manual intervention.

Train Inauguration and Topology Discovery

One of the most critical functions defined by the standard is “train inauguration.” This is the process by which the network initializes itself and learns the train’s current composition.

• Train Topology Discovery Protocol (TTDP): EN 61375-2-5 specifies the TTDP, a protocol responsible for discovering the sequence of vehicles, their orientation (direction), and the status of the interconnecting links.
• Dynamic Address Assignment: During inauguration, network nodes are assigned dynamic addresses based on their position within the train. This ensures that communication can be correctly routed regardless of how the train is assembled.
• End-of-Train Detection: The protocol reliably identifies the first and last vehicles of the train, which is essential for managing the linear network topology and preventing data loops.

Redundancy and High Availability

The railway environment is harsh, and connections can be subject to mechanical stress, vibration, and intermittent failures. The ETB standard incorporates robust mechanisms for redundancy.

• Link Redundancy: As mentioned, dual Ethernet links between cars are standard. If one link fails, a network management protocol like the Rapid Spanning Tree Protocol (RSTP) or a similar mechanism ensures that traffic is automatically rerouted through the alternate link with minimal interruption.
• Fault Tolerance: The network is designed to tolerate the failure of a single inter-vehicle link or even a node without causing a complete loss of communication across the train. The system gracefully degrades, maintaining connectivity where possible.

Communication Protocols and Physical Layer

The standard builds upon established Ethernet technology but specifies adaptations for railway use.

• Data Services: The ETB is designed to carry non-vital, high-bandwidth data. It supports standard IP-based traffic, allowing various applications like video streaming, VoIP, and passenger Wi-Fi to coexist on the same network.
• Physical Connectors and Cabling: To withstand the rail environment, the standard typically implies the use of ruggedized hardware. This includes M12 X-coded connectors, which provide a secure, vibration-resistant connection, and shielded Ethernet cables rated for wide temperature ranges and high electromagnetic interference (EMI) immunity.
• Managed Switches: The nodes within the ETB are managed Ethernet switches. This allows for the implementation of necessary protocols like TTDP, Quality of Service (QoS) for prioritizing traffic, and network monitoring via SNMP.

ETB in Context: Comparison with Wire Train Bus (WTB)

The Ethernet Train Backbone is often seen as the modern successor or complement to the older Wire Train Bus (WTB), defined in EN 61375-2-3. While both serve as train-wide communication networks, their capabilities and intended applications are vastly different.

Role within the Modern Train Communication Network

In a contemporary rolling stock design, the ETB does not typically replace the WTB. Instead, they operate in parallel to form a complete communication solution. The WTB continues to handle the deterministic, safety-critical train control functions, while the ETB provides a high-capacity backbone for all other data-intensive services. The ETB connects multiple Ethernet Consist Networks (ECNs), which are the local area networks within each vehicle or a fixed set of vehicles, allowing seamless IP communication from end to end of the train.

Conclusion

The EN 61375-2-5 standard is a foundational element for the digitalization of modern railways. By defining a robust, high-bandwidth, and self-configuring Ethernet Train Backbone, it enables the deployment of advanced passenger services and operational systems that were not possible with legacy fieldbus technologies. Its focus on standardized Ethernet, combined with railway-specific requirements for redundancy and environmental hardening, ensures that the ETB provides a reliable and future-proof communication platform for the rolling stock of today and tomorrow.