Metro carriage material toxicity analysis evaluates the toxic gas emissions of interior and structural materials under standardized fire conditions, ensuring passenger safety in the confined, high-occupancy environment of urban rail vehicles. This article explains the key standards—such as EN 45545-2, ISO 5659-2, and GB/T specifications—along with typical test samples, testing items, methods, and compliance requirements used by accredited third-party laboratories.
Technical Overview
Metro carriages operate in tunnels and underground stations where egress routes are long, ventilation is limited, and passenger density is extremely high. In the event of a fire, statistical studies consistently show that the majority of casualties result from inhalation of toxic combustion products—primarily carbon monoxide (CO) and hydrogen cyanide (HCN)—rather than from heat or direct flame contact. Because modern metro vehicles contain a large share of polymeric and composite materials, the toxicological behavior of these materials under thermal decomposition is a decisive safety parameter.
Toxicity analysis of metro carriage materials is the systematic evaluation of the identity and concentration of hazardous gases released when a material is exposed to defined irradiance levels and ignition scenarios. The results are converted into standardized indices—most notably the Conventional Index of Toxicity (CIT) defined in EN 45545-2—and compared against acceptance limits that depend on the vehicle design category and operating conditions. In Europe and many international metro projects, EN 45545-2 hazard levels HL1 to HL3 apply, while Chinese rolling stock projects additionally reference TB/T series railway standards and GB/T test methods, and North American or Japanese specifications may call upon UL, ASTM, or JIS flammability and smoke procedures.
The overall objective is twofold: to verify that each material, component, and assembly delays the onset of incapacitating gas concentrations long enough to allow safe evacuation, and to provide validated input data for fire hazard modeling of the complete vehicle.
Testing Samples
Testing must be performed on specimens that represent the material exactly as installed, including production formulation, thickness, color, surface treatment, and any laminate or coating system. Samples are normally conditioned to equilibrium at 23 °C and 50% relative humidity before testing, and the maximum end-use thickness is generally selected as the worst case.
Structural and Interior Panel Materials
- Glass-fiber-reinforced plastic (GRP) and phenolic composite panels for sidewalls, ceilings, and partition walls
- Aluminum honeycomb laminates and sandwich panels with decorative laminates or coatings
- Polycarbonate and PMMA glazing, light diffusers, and transparent covers
- Painted, laminated, or powder-coated surfaces applied to metal substrates
- Driver cab consoles, control panels, and electrical cabinet housings
Seating and Textile Products
- Flexible polyurethane seat foams and molded foam cores
- Upholstery fabrics, moquette, and knitted or woven cover materials
- Seat shells, headrests, and armrests made of thermoplastics or composites
- Hook-and-loop fasteners, piping cords, and decorative trim tapes
Flooring and Elastomeric Components
- PVC and rubber floor coverings, including wear layers and printed decor layers
- Door sill plates, floor nosings, and edging profiles
- Rubber and thermoplastic elastomer seals, gaskets, gangway bellows, and diaphragms
- Hand straps, grab handles, and other passenger-contact elastomeric parts
Cables, Adhesives, and Coatings
- Low-smoke zero-halogen (LSZH) cable sheathing and wire insulation compounds
- Structural adhesives, flooring adhesives, sealants, and potting compounds
- Paints, primers, varnishes, and thin decorative films
- Thermal and acoustic insulation materials installed within the car body
Testing Items
Toxicity analysis is not performed in isolation; it forms part of an integrated fire safety qualification package. The core testing items for metro carriage materials typically include the following:
- Identification and quantification of combustion gases: CO, CO2, HCN, HCl, HBr, HF, SO2, and NOx
- Conventional Index of Toxicity (CIT_G for listed products, CIT_NLP for non-listed products) calculated per EN 45545-2
- Smoke density parameters: specific optical density Ds(4), Ds max, and accumulated smoke volume VOF4 per ISO 5659-2 or GB/T 8323
- Heat release properties: peak and total heat release rate, and MARHE derived from ISO 5660-1 cone calorimeter data
- Flame spread, ignition, and flame propagation behavior under radiant and small-flame exposure
- Limiting oxygen index (LOI) and vertical or horizontal burning classification such as UL 94 V-0
- Glow-wire ignitability and flammability for electrotechnical components
- Halogen content and heavy-metal screening for material composition verification
- Volatile organic compound (VOC) and formaldehyde emission testing for interior air quality, per the ISO 16000 series and railway harmful-substance limit standards such as TB/T 3139
Acceptance limits for each item depend on the applicable requirement set of EN 45545-2 (for example R1 for interior vertical surfaces and R6 for seating) and on the hazard level HL1, HL2, or HL3, which is assigned according to the train design and operation category. Tunnel-operated metro vehicles generally attract the most demanding requirement levels.
Testing Methods
A tiered testing strategy is used, combining smoke generation, gas analysis, and supplementary fire performance tests so that the toxic hazard of each material is characterized under realistic decomposition conditions.
Smoke Generation and Gas Sampling — ISO 5659-2
The single-chamber smoke test per ISO 5659-2 (or the equivalent GB/T 8323 method) exposes 75 mm × 75 mm specimens to radiant fluxes of 25 kW/m² and, where required, 50 kW/m², both with and without piloted ignition. Light transmission measurements yield the smoke density parameters Ds(4), Ds max, and VOF4, while the chamber atmosphere is simultaneously sampled for toxic gas analysis. This test provides the gas concentration data that feed directly into the CIT calculation.
Gas Analysis and CIT Calculation — EN 17084 / ISO 19021
Gas sampling from the ISO 5659-2 chamber is analyzed by Fourier-transform infrared spectroscopy (FTIR) in accordance with EN 17084 and the corresponding ISO 19021 methodology. The measured concentrations of the eight regulated gases are divided by their reference concentrations defined in EN 45545-2, and the normalized values are summed to produce the Conventional Index of Toxicity. For non-listed products, specimens are evaluated at both irradiance levels and the results are combined according to the calculation rules of the standard. A lower CIT indicates a lower toxic hazard, and the value must not exceed the limit assigned to the relevant requirement set and hazard level.
Heat Release and Flame Spread Methods
- ISO 5660-1 / ASTM E1354 cone calorimeter testing at 50 kW/m² to determine heat release rates and MARHE
- ISO 5658-2 lateral flame spread on panels to determine the flame spread index
- ISO 9239-1 radiant panel testing for flooring materials
- ISO 11925-2 small-flame ignition test and ASTM D2863 limiting oxygen index
- UL 94 vertical and horizontal burning classifications for plastics and components
Component-Level and Supplementary Methods
Electrotechnical parts are assessed using IEC 60695-2-10 and IEC 60695-2-12 glow-wire methods, while cables are evaluated with IEC 60332-1 flame propagation and IEC 61034-2 smoke density tests. Halogen and sulfur content can be verified by combustion-ion chromatography per EN 14582, and heavy metals by IEC 62321 procedures. For fire hazard engineering studies, fractional effective dose (FED) and fractional effective concentration (FEC) calculations per ISO 13344 and ISO 13571 translate material gas yields into tenability limits for occupant evacuation modeling.
Applications
Toxicity analysis of metro carriage materials supports every phase of a rolling stock program, from early material selection through series production and fleet modernization.
- New vehicle qualification: demonstrating compliance of all interior and exterior materials with the fire safety specification cited in the purchase contract
- Supplier control: incoming material verification and periodic production surveillance to confirm that flame retardancy and toxicity performance remain stable
- Export projects: aligning test programs with EN 45545-2, GB/T, UL, ASTM, IEC, and JIS requirements demanded by different markets
- Research and development: screening halogen-free flame retardant systems, low-fume composites, and LSZH compounds to achieve lower CIT values
- Retrofit and refurbishment: requalification of seats, flooring, panels, and coatings when legacy fleets are modernized
- Fire safety engineering: supplying validated gas yield and smoke data for computational fluid dynamics and evacuation simulations of tunnels and stations
Accredited third-party laboratories issue the test reports and, where required, formal conformity documentation that vehicle builders submit for type approval by the operator or safety authority.
FAQ
What is the Conventional Index of Toxicity (CIT)?
CIT is a single-number index defined in EN 45545-2 that expresses the combined toxic potency of the gases released by a material in the ISO 5659-2 smoke chamber. It is calculated from the ratio of measured gas concentrations to reference concentrations for CO, CO2, HCN, HCl, HBr, HF, SO2, and NOx. A CIT below the specified limit demonstrates that the material's combustion gases remain below incapacitating thresholds during the critical early phase of a fire.
Which standards apply to a metro project?
For vehicles procured to European practice, EN 45545-2 governs material requirements, supported by EN 17084 for gas analysis. Projects in China combine TB/T railway standards for flame retardancy and harmful substance limits with GB/T test methods. Where customer specifications reference UL, ASTM, IEC, or JIS procedures, the laboratory builds a combined test matrix covering all cited standards.
What samples should be submitted for testing?
Samples must represent the end-use product: production material, final thickness, final color, and complete coating or lamination systems. Panels are usually cut into 75 mm × 75 mm coupons for smoke chamber testing, while foams and textiles are tested at maximum installed thickness. Sufficient material should be supplied for conditioning and for repeat testing at multiple irradiance levels.
Do adhesives and small components require separate testing?
Yes. Adhesives, sealants, coatings, and fastening systems contribute to the overall toxic load and are tested either individually or as part of the finished assembly. EN 45545-2 provides a dedicated route for non-listed products, requiring assessment at both 25 kW/m² and 50 kW/m² irradiance with the CIT_NLP calculation.
How are fire toxicity and interior air quality testing related?
They address different hazards. Fire toxicity testing evaluates gases released during combustion or pyrolysis in a fire scenario. Interior air quality testing, using ISO 16000 series chamber methods and harmful-substance limit standards such as TB/T 3139, measures VOC and formaldehyde emissions at normal room temperature to protect passengers from chronic exposure during everyday operation. Comprehensive material qualification covers both aspects.
How long does a typical test program take?
A full qualification program for a metro carriage interior material set typically spans several weeks, because it includes specimen conditioning, multiple irradiance conditions, gas analysis, and calculation of indices for each requirement set. Early engagement with an accredited third-party laboratory allows test matrices to be optimized, reducing both cost and lead time while ensuring complete coverage of the applicable hazard levels.