Metro vehicle material combustion testing evaluates the ignitability, flame spread, heat release, smoke density and toxic gas emission of materials installed in subway and metro car interiors, exteriors and cabling. Guided by EN 45545-2, TB/T 3138 and related ISO, IEC, EN and GB standards, accredited third-party laboratories classify materials against hazard level requirements to verify fire safety compliance for underground passenger operation. This article explains the testing scope, sample types, test items, methods and typical applications in detail.
Technical Overview
Metro vehicles operate in enclosed tunnel environments with high passenger density, restricted evacuation routes and limited access for emergency services. Under these conditions, even a small material-related fire can rapidly produce dangerous levels of heat, smoke and toxic gases. Combustion testing of metro vehicle materials is therefore a mandatory part of vehicle design, type approval and production control.
The internationally recognized framework is EN 45545-2 (Railway applications — Fire protection on railway vehicles — Requirements for fire behaviour of materials and components). EN 45545-1 assigns each vehicle to an operation category and design category, which together determine the hazard level: HL1 (lower risk), HL2 (medium risk) or HL3 (highest risk). Metro trains running in single-track tunnels with high passenger loads are typically classified as HL3, the most stringent level. Each installed product category is then mapped to a requirement set (R1 to R28), and every material must achieve the specified classification parameters for the applicable hazard level.
In China, railway and metro projects commonly reference TB/T 3138 together with GB/T test methods, while legacy European national standards such as DIN 5510-2, BS 6853 and NF F 16-101 have largely been superseded by EN 45545. Testing is performed by accredited third-party laboratories operating under ISO/IEC 17025 quality systems, and results are reported as classification reports that support vehicle homologation and tender compliance.
Testing Samples
Interior Panels and Finishes
Wall, ceiling and partition panels manufactured from glass-fiber reinforced plastics (GRP), aluminum honeycomb composites, phenolic laminates, melamine-faced boards and polycarbonate glazing. These large-area products strongly influence flame spread and smoke development inside the passenger saloon.
Flooring Systems
PVC or rubber floor coverings, floor adhesives, plywood or aluminum substrates, and slip-resistant overlays. Flooring is evaluated as a complete built-up assembly because the substrate and adhesive affect the burning behavior of the surface layer.
Seats and Upholstery
Seat shells, cushioning foams, textile and artificial leather covers, headrest covers and armrest components. Upholstered assemblies require particular attention because foams can contribute significantly to heat release and smoke toxicity.
Cables and Wiring
Power cables, control and data cables, harnesses, conduits, cable trays and connector materials. Cables run the entire length of the train and through vehicle interfaces, so they are governed by dedicated vertical flame propagation tests.
Elastomers and Seals
Door and window gaskets, gangway bellows, rubber hoses, sealing profiles and anti-vibration mounts. Elastomers are evaluated for oxygen index, flame spread and smoke generation.
Insulation and Acoustic Materials
Mineral wool, acoustic foams, nonwoven thermal wraps and underfloor insulation pads installed between the car body shell and interior linings.
Coatings, Adhesives and Markings
Interior and exterior paints, decals, lacquers, structural adhesives and printed films, tested individually or as part of the coated substrate system.
Structural Composites and Equipment Housings
Cab front masks, FRP fairings, equipment covers, air ducting and HVAC components made from thermoset or thermoplastic composites.
Before testing, specimens are cut to the dimensions and orientation specified by each test method, prepared at the end-use thickness, and conditioned to equilibrium at 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with ISO 291. Most methods require three to six replicate specimens per test condition, and the worst-case configuration of the product is normally selected.
Testing Items
Depending on the applicable requirement set and hazard level, metro vehicle materials are assessed for the following fire performance characteristics:
- Ignitability under small flame exposure or hot-surface conditions
- Flame spread distance and rate, both lateral and vertical
- Heat release rate, total heat release and maximum average rate of heat emission (MARHE)
- Smoke density, expressed as specific optical density (Ds max), Ds(4) and the cumulative smoke value VOF4
- Toxic gas emission, expressed as the conventional index of toxicity (CIT), based on CO, CO2, HCN, HCl, HBr, HF, NOx and SO2 concentrations
- Limited oxygen index (LOI), the minimum oxygen concentration that supports flaming combustion
- Formation of flaming droplets or burning debris
- After-flame time, afterglow time and damaged length after flame removal
- Glow-wire flammability index (GWFI) and glow-wire ignition temperature (GWIT)
- Vertical flame propagation of single and bunched cables
- Critical heat flux at extinguishment (CFE) for lateral spread and critical radiant flux (CHF) for flooring
- Material classification levels such as UL 94 V-0, V-1, V-2 or HB for electronic component enclosures
Testing Methods
EN 45545-2 System Approach
EN 45545-2 defines test methods T01 to T17 and assigns them to product requirement sets R1 to R28. For each requirement set, the material must meet classification parameters for ignitability, spread of flame, heat release, smoke density and toxicity that correspond to hazard levels HL1, HL2 or HL3. Because metro operation underground is classified at the highest risk level, most interior materials are qualified to HL3 limits, which impose the lowest permissible smoke densities and toxicity indices.
Smoke Density and Toxicity (ISO 5659-2 with FTIR)
Specimens are exposed in a closed smoke chamber at radiant fluxes of 25 kW/m² and, for higher hazard levels, 50 kW/m², with and without piloted ignition. Transmittance measurements yield Ds max and VOF4. In parallel, evolved gases are analyzed continuously by FTIR spectroscopy, and the conventional index of toxicity (CIT) is calculated from the measured gas concentrations, reference concentrations and material mass as defined in EN 45545-2 Annex C. The equivalent Chinese method is GB/T 8323.2.
Heat Release (ISO 5660-1 Cone Calorimeter)
The specimen is irradiated at 50 kW/m² in an open combustion system. The test records peak heat release rate, total heat release and MARHE, which integrates the heat emission profile over the first minutes of exposure. MARHE is the key acceptance parameter in EN 45545-2 for interior surface products.
Flame Spread and Ignition (ISO 5658-2, ISO 11925-2, ISO 871)
ISO 5658-2 measures lateral flame spread along a horizontally mounted specimen exposed to an inclined radiant panel, reporting flame spread distance and critical flux at extinguishment. ISO 11925-2 applies a small premixed flame for 15 or 30 seconds to assess direct ignitability, flame spread and flaming droplets. ISO 871 determines the piloted ignition temperature using a hot-air furnace, relevant for materials exposed to hot surfaces.
Limited Oxygen Index (EN ISO 4589-2)
The LOI test determines the minimum oxygen concentration in a nitrogen-oxygen mixture that just supports candle-like burning of a vertically mounted specimen. It is widely used for quality control of polymers, elastomers and textiles, and is also required by TB/T 3138. The equivalent Chinese method is GB/T 2406.2.
Flooring (EN ISO 9239-1)
Floor coverings are tested in the horizontal position with an inclined radiant heat source and a pilot flame. The test determines the critical heat flux at which flame front propagation extinguishes, which must exceed the limit defined for the applicable hazard level.
Component-Level Tests (IEC 60695 Series)
Electrotechnical components, enclosures and small parts are evaluated using IEC 60695-2-10 for the glow-wire flammability index, IEC 60695-2-12/60695-2-13 for glow-wire ignition temperature, IEC 60695-11-10 for 50 W vertical and horizontal burning (the basis of UL 94 classifications) and IEC 60695-11-5 for needle-flame testing. These methods correspond to test methods T11 and T12 of EN 45545-2.
Cable Fire Tests (EN 60332 Series and EN 50305)
Chinese and Legacy Standard Frameworks
For domestic metro projects, TB/T 3138 specifies fire-resistance requirements for locomotive and rolling stock materials, using GB/T methods for oxygen index, horizontal and vertical burning (GB/T 2408), smoke density and toxicity. Customers in some export markets previously qualified materials to DIN 5510-2 (S, SR and ST classes), BS 6853 (category Ia to III) or NF F 16-101 (I, M and F classes); these national standards have been withdrawn in favor of EN 45545-2, although some legacy fleets still reference them. North American transit programs typically build on ASTM E162 flame spread index, ASTM E662 NBS smoke chamber and ASTM E648 flooring critical radiant flux.
Applications
- Type approval and homologation of new metro trains, including HL3 compliance packages for underground operation
- Tender and bid documentation demonstrating material compliance with project fire safety specifications
- Material substitution and qualification during fleet refurbishment and interior retrofit programs
- Supplier change control and periodic surveillance of series production materials
- Research and development of flame-retardant polymers, composites, foams and coatings with optimized smoke and toxicity performance
- Fire risk assessments and safety case support for metro operators and vehicle integrators
- Failure analysis and forensic investigation following material or component fire incidents
FAQ
Which hazard level applies to metro vehicles?
Hazard levels are assigned under EN 45545-1 from the combination of operation category and design category. Metro vehicles running in tunnels with high passenger numbers and automated or crewless operation are normally classified as HL3, requiring the strictest limits for flame spread, smoke density and toxicity.
What is the difference between EN 45545-2 and TB/T 3138?
EN 45545-2 is the harmonized European railway fire standard based on a hazard-level system, while TB/T 3138 is the Chinese railway industry standard specifying fire-resistance requirements for rolling stock materials. Both rely on similar fundamental measurements such as oxygen index, smoke density, toxicity and flame spread, and many projects require dual qualification by accredited third-party laboratories.
What does CIT mean and why is it important?
CIT is the conventional index of toxicity calculated from FTIR gas measurements during ISO 5659-2 smoke chamber testing. It compares the emitted quantities of CO, CO2, HCN, HCl, HBr, HF, NOx and SO2 against reference exposure limits. Toxic smoke is the primary cause of casualties in tunnel fires, so CIT limits are especially strict at HL3.
What is VOF4?
VOF4 is the cumulative integral of specific optical density over the first four minutes of a smoke chamber test. It characterizes how quickly a material obscures vision during the critical early phase of evacuation.
How many specimens are required for testing?
Most methods require three to six conditioned specimens per test condition, plus replacement specimens in case of borderline results. Flooring and composite assemblies must be tested in the end-use build-up, including adhesives and substrates.
Do coatings, adhesives and inks need separate testing?
Yes. They are qualified either as individual materials or as part of the finished coated system, depending on the applicable requirement set. Thin layers can influence ignitability, dripping and smoke behavior of the underlying substrate.
Are older reports to DIN 5510-2 or BS 6853 still acceptable?
These national standards have been withdrawn in Europe and replaced by EN 45545. Reports may still be referenced for legacy fleets or transitional contracts, but new metro vehicle projects generally require classification to the current EN 45545-2 framework.
