What is EN 50155? The Definitive Technical Guide (2021 Edition)

EN 50155 is the European standard governing all electronic equipment installed on railway rolling stock. Published by CENELEC and currently in its EN 50155:2021 edition (which superseded EN 50155:2017 on 17 August 2021), it defines mandatory requirements for design, documentation, testing, and integration of onboard electronic systems — from a simple passenger information display to a complex Train Control and Management System (TCMS).

Without EN 50155 certification, no electronic hardware can be legally deployed on European rail vehicles, and increasingly, operators worldwide treat it as the de facto global benchmark for railway electronics reliability.

1. Scope: What Does EN 50155 Cover?

The standard applies to all electronic equipment installed on rail vehicles performing functions including:

• Control and regulation (TCMS, ATP, ATO)
• Protection and safety monitoring
• Diagnostics and condition monitoring
• Energy supply and power conversion
• Communication systems (GSM-R, LTE, Wi-Fi)
• Passenger information systems (PIS)
• Sensor systems (speed, current, voltage, LiDAR)

Important scope boundaries:

• Complete semiconductor drive units and power converters → covered by EN 61287-1
• Software requirements for onboard equipment → covered by EN 50657
• Functional safety integrity levels → covered by EN 50129
• EMC requirements → covered by EN 50121 series
• Shock and vibration testing → referenced standard IEC 61373

2. Standard Version History

3. Why the Railway Environment Demands a Dedicated Standard

Consumer or industrial electronics standards (IEC 60068, MIL-STD-810) do not adequately address the specific stressors present in railway operation. A train subjects onboard electronics to a uniquely hostile combination of factors:

3.1 Mechanical Stress

Wheel-rail interaction generates broadband vibration across 0–150 Hz. Bogie-mounted equipment faces significantly higher acceleration levels than body-mounted equipment. Track irregularities, switches, and level crossings create impulsive shock loads. Buffer impacts during shunting operations can reach 3g peak acceleration.

3.2 Electrical Environment

The traction power supply — whether 750 V DC third rail, 1500 V DC overhead, 3 kV DC, or 25 kV AC — is inherently unstable. Voltage fluctuates during acceleration, regenerative braking, and pantograph transitions between supply sections. Auxiliary converters feeding 24 V, 48 V, or 110 V DC bus rails to control electronics must maintain output stability across a wide input window.

3.3 Thermal Environment

Equipment in driver’s cabs, exterior sensor housings, and roof-mounted antennas faces ambient temperatures from below -40°C in Nordic winters to above +70°C in summer sun loading. Engine room and electrical cabinet temperatures can exceed +85°C during sustained high-load operation.

3.4 Atmospheric Conditions

Tunnels expose equipment to sudden pressure changes, condensation, and high humidity. Coastal and industrial routes introduce salt mist and chemical pollutants. Altitude variations on mountain railways affect convective cooling efficiency.

4. Operating Temperature Classes (EN 50155:2021)

EN 50155:2021 defines six operating temperature classes. The default class for most equipment is OT3. Every certified device must state its class clearly in product documentation.

Start-up temperature requirement: All equipment must be capable of starting and achieving full operational status after exposure to the minimum temperature of its class for a minimum of 10 minutes.

Rapid temperature variation (introduced EN 50155:2017): Equipment mounted outside the vehicle body must withstand rapid temperature changes — such as a train entering a heated tunnel from a -30°C exterior environment — without condensation-induced failure.

5. Shock and Vibration Testing (IEC 61373)

EN 50155 references IEC 61373:2010 for mechanical testing. Equipment is classified by its mounting position, which determines the vibration and shock levels it must survive:

Testing consists of three phases: functional vibration tests (equipment operating during vibration), endurance vibration tests (long-duration exposure), and shock tests (simulating buffer impacts and emergency braking).

6. Power Supply Requirements

The power supply section is one of the most technically demanding areas of EN 50155. It addresses the instability of the onboard DC bus supplied from the auxiliary converter.

6.1 Nominal Voltage Levels

Common onboard DC bus voltages: 24 V, 48 V, 72 V, 96 V, 110 V. Equipment must operate correctly across a defined voltage window — typically ±25% of nominal — under steady-state conditions.

6.2 Voltage Dip and Interruption Classes

EN 50155:2021 revised the supply voltage interruption classes. Equipment must survive and recover from supply interruptions without data loss or dangerous failure states:

Key 2021 addition: The standard now explicitly prohibits loss of significant stored data during supply interruptions. Safety-critical data, event logs, and configuration parameters must be preserved through hold-up capacitance or non-volatile storage.

6.3 Surge and Transient Requirements

• Sustained overvoltage: Equipment must survive 1.4× nominal voltage for up to 1 second
• Transient spikes: Equipment must withstand fast transients per EN 61000-4-4 (EFT/Burst testing)
• Reverse polarity protection: Mandatory for equipment connected to battery-backed supplies

7. Performance Criteria During Testing

EN 50155:2021 introduced a third performance criterion, expanding the original two-criterion framework:

8. Humidity, Ingress Protection, and Atmospheric Conditions

EN 50155 references EN 50125-1 for the environmental conditions railway equipment must withstand. Key humidity requirements:

• Relative humidity: Up to 95% non-condensing for interior equipment; condensing conditions must be handled by appropriate IP rating or conformal coating
• IP rating: Equipment mounted in sealed enclosures may use the cabinet’s IP rating; standalone devices must achieve the required IP class independently (typically IP54 minimum for non-enclosed equipment)
• Altitude: Standard coverage up to 1800 m; equipment for high-altitude routes (Swiss Alps, Tibetan Plateau) requires derating calculations for reduced air cooling efficiency

9. EN 50155 and Related Standards: The Full Compliance Framework

EN 50155 does not operate in isolation. A complete compliance framework for rolling stock electronics involves multiple interconnected standards:

10. Modern Applications: EN 50155 in Smart Train Systems

As rail vehicles evolve into intelligent transport platforms, EN 50155 certification requirements extend to a new generation of onboard technology:

10.1 Edge Computing and AI Hardware

Onboard computers processing real-time data for predictive maintenance, obstacle detection, and autonomous train operation must achieve OT5 or OT6 class ratings if roof-mounted, and survive Category 1B vibration profiles. Industrial-grade GPUs and AI accelerators require custom thermal management solutions to meet EN 50155 requirements.

10.2 LiDAR and Perception Sensors

Roof-mounted LiDAR units for obstacle detection and platform gap measurement face the harshest thermal and vibration environment on the train. They require OT5 certification (-40°C to +85°C), Category 1B shock and vibration testing, and IP67 or higher ingress protection.

10.3 Ethernet Train Backbone (ETB) and Wi-Fi Systems

Passenger Wi-Fi access points and ETB (Ethernet Train Backbone) switches conforming to EN 50701 (cybersecurity) must simultaneously meet EN 50155 environmental requirements. High-bandwidth routers operating at elevated ambient temperatures require careful thermal derating.

10.4 IoT Gateways for Predictive Maintenance

Gateways aggregating data from vibration sensors, oil analysis units, and brake pad wear monitors must meet EN 50155 power interruption class S4 or higher — ensuring data integrity is maintained when the vehicle’s auxiliary power supply drops during pantograph transitions.

10.5 ETCS Onboard Units

European Train Control System (ETCS) onboard units, which are safety-critical, must comply with both EN 50155 (environmental) and EN 50129 (functional safety, SIL 4). This dual compliance requirement significantly increases certification cost and timeline.

11. EN 50155 Certification Process: What Manufacturers Must Do

Achieving EN 50155 compliance requires a structured engineering and documentation process:

1. Define the equipment class: Determine mounting category, operating temperature class, vibration category, IP rating, and supply voltage interruption class based on the intended installation location
2. Design for compliance: Select components rated for the required temperature range; design power supply hold-up circuitry; apply conformal coating if humidity exposure is expected
3. Conduct type testing: Submit the equipment to an accredited test laboratory for environmental testing per IEC 61373, EN 50125-1, and the power supply sections of EN 50155
4. Prepare technical documentation: Produce a Technical File including design rationale, bill of materials, test reports, and compliance declaration
5. Issue Declaration of Conformity: The manufacturer issues a DoC referencing EN 50155:2021 and all normatively referenced standards
6. Maintain certification: Any significant design change (component substitution, PCB revision, firmware update affecting hardware behavior) requires re-assessment

12. Frequently Asked Questions

Is EN 50155 mandatory for railway equipment in Europe?

EN 50155 is a harmonized European standard. While it is not itself a legal regulation, the EU Railway Interoperability Directive and associated Technical Specifications for Interoperability (TSIs) reference it directly. Rail operators and vehicle integrators contractually require EN 50155 compliance, making it effectively mandatory for any equipment intended for European railway deployment.

What is the difference between EN 50155:2017 and EN 50155:2021?

The 2021 edition introduced a third performance criterion (Criterion C) allowing degradation with mandatory data preservation, revised the supply voltage interruption class table, and clarified requirements arising from unresolved technical comments in the 2017 balloting process. Equipment certified to the 2017 edition required re-assessment against the 2021 edition after August 2021.

Does EN 50155 cover software?

No. Software requirements for onboard railway equipment were split into a separate standard — EN 50657 — starting with the 2017 edition. EN 50155:2021 covers hardware environmental requirements only; EN 50657 addresses software development lifecycle, coding standards, and verification requirements.

What is the global equivalent of EN 50155?

The IEC equivalent is IEC 60571, which covers similar scope for markets outside Europe. North American projects often reference IEEE 1475 alongside IEC 60571. Most international projects now accept EN 50155 compliance as meeting or exceeding IEC 60571 requirements.

Which temperature class is required for exterior-mounted sensors?

Equipment mounted on the exterior of the vehicle body — including roof-mounted antennas, pantograph monitoring cameras, and LiDAR units — typically requires OT5 class (-40°C to +85°C). For operation in arctic conditions (Scandinavia, Canada, Russia), OT6 class (-50°C to +85°C) may be specified.

Can consumer-grade components be used in EN 50155-certified equipment?

Consumer-grade components (rated 0°C to +70°C) are generally unsuitable for EN 50155-certified designs beyond OT1 class. Industrial-grade components (typically -40°C to +85°C) are the minimum acceptable for most railway applications. For safety-critical ETCS or ATP functions, component selection must also satisfy EN 50129 traceability and FMEA requirements.

Conclusion

EN 50155:2021 is the engineering foundation on which all reliable railway electronic systems are built. Its requirements — spanning thermal extremes, mechanical shock, power supply instability, and data integrity — reflect decades of operational experience with the uniquely demanding railway environment.

For system integrators, the standard provides a clear compliance pathway. For operators, it provides assurance that certified equipment has been rigorously validated for the conditions it will face in service. And for the railway industry as a whole, it provides the harmonized technical language that makes interoperability across Europe’s diverse rail networks possible.

As trains become increasingly intelligent — incorporating AI-driven maintenance systems, autonomous driving technology, and high-bandwidth passenger connectivity — EN 50155 compliance becomes not a bureaucratic hurdle but a genuine engineering benchmark: proof that the hardware is ready for the railway.