Metro coupling passages (gangways) connect adjacent vehicle bodies and rely heavily on rubber-coated fabrics, elastomeric seals, adhesives and coatings that must satisfy stringent fire-safety requirements in confined underground environments. Combustion testing of these materials verifies flame retardancy, smoke production and combustion-gas toxicity against standards such as EN 45545-2, NFPA 130 and TB/T 3138. This article explains the sample types, test items, test methods and acceptance logic used by accredited third-party laboratories to qualify coupling passage materials for metro vehicle projects.
Technical Overview
A metro coupling passage, also called a gangway or vestibule diaphragm, is the flexible corridor installed between adjacent vehicle bodies that allows passengers and crew to move between cars while the train is in motion. A typical unit consists of a folding bellows made of rubber-coated fabric, an aluminium or stainless steel frame system, a tread-plate floor mechanism, sealing profiles, roof and side panels, and a variety of adhesives, foams and surface coatings. Because the passage sits directly inside the passenger compartment and the vehicle operates in long, confined tunnels, its polymeric components are among the most fire-critical materials on a metro train.
Fire safety engineering for metro vehicles rests on three principles: prevent ignition, limit flame propagation, and control the generation of smoke and toxic gases. Statistics from tunnel fire incidents consistently show that smoke and toxic combustion products, rather than flame itself, are the primary causes of casualties. Combustion testing of coupling passage materials therefore evaluates limiting oxygen index, ignitability, flame spread, heat release, smoke density and the toxicity of combustion gases. Compliance is demonstrated against regional frameworks such as EN 45545-2 in Europe, NFPA 130 in North America, legacy national railway standards including BS 6853, DIN 5510-2 and NF F 16-101, and the Chinese railway standards TB/T 3138 and TB/T 3139 together with the corresponding GB/T method standards.
Under EN 45545-2, every material and component is assigned to a product category (interior linings, elastic components, exterior parts, mechanical parts and electrical parts) that determines the applicable requirement set, and the material must meet the criteria for the hazard level defined in EN 45545-1:
- HL1 – lower-risk operation with short evacuation times and easy escape routes.
- HL2 – vehicles operating partly in tunnels with longer evacuation paths.
- HL3 – the most demanding level, typically applied to driverless or automatically operated metro trains running in long tunnels, which is the usual case for modern urban rail lines.
Testing Samples
Rubber-Coated Bellows Fabric
The wall and roof sheets of the folding bellows are usually EPDM, chloroprene rubber or NR/SBR compounds, flame-retarded and calendered onto polyester or glass-fibre fabric; PVC and TPU membranes are also used on some designs. Specimens are cut from production-grade sheet in the final supplied thickness, including any seams, welds and surface finishes, and are conditioned to constant mass at 23 °C and 50% relative humidity before testing.
Elastomeric Sealing and Molded Components
Sealing profiles, frame gaskets, rubber draw plates, bellows hinges and resilient buffers are tested in their final elastomer formulation. These parts are often thin-walled and flexible, so laboratories follow the special specimen-mounting provisions of the relevant method standards for non-self-supporting materials.
Floor Components
Gangway floor membranes, wear layers, tread-plate coatings and anti-slip finishes are treated as flooring products. They are evaluated with radiant-panel flame-spread methods in addition to the standard flammability and smoke tests, because floor coverings contribute to horizontal fire growth between vehicle ends.
Frames, Panels, Adhesives and Coatings
Metallic frames and plates are non-combustible, but the paints, powder coatings, adhesives, sealants, insulating foams and polymeric bushes mounted on them are combustible and must be qualified. Where a finish cannot be separated from its substrate, testing is performed on the coated system as installed.
Complete Coupling Passage Assemblies
For type approval, component-level data may be complemented by reference or full-scale fire testing of the assembled gangway, in order to verify the behaviour of interfaces, fasteners and mixed-material junctions that cannot be represented by small specimens alone.
Testing Items
A typical qualification scope for coupling passage materials includes the following items, each mapped to a specific acceptance criterion in the governing standard:
- Limiting oxygen index (LOI)
- Ignitability and small-flame behaviour: after-flame time, afterglow time, damaged length
- Flame spread rate and flame spread index
- Heat release rate, total heat release and average rate of heat emission (MARHE)
- Melting behaviour and flaming droplets or debris
- Smoke production: specific optical density Ds(1.5), Ds(4.0), Ds max and the cumulative value VOF4
- Smoke toxicity: conventional toxicity index (CIT) and concentrations of CO, CO2, HCN, HCl, HF, HBr, SO2 and NOx
- Mass loss rate and horizontal or vertical burning rate
Testing Methods
Limiting Oxygen Index — ISO 4589-2 / GB/T 2406.2 / ASTM D2863
The LOI test determines the minimum oxygen concentration in a flowing oxygen–nitrogen mixture that will just support candle-like burning of a vertically mounted specimen. It is the primary screening test in TB/T 3138 and remains a verification method within EN 45545-2. Flame-retardant bellows fabric is typically specified with an LOI of 28% or higher.
Small-Flame Ignition — ISO 11925-2, GB/T 2408, IEC 60695-11-10, UL 94
Specimens are exposed to a standard pilot flame for 15 s or 30 s. Acceptance depends on the flame tip not reaching a marked gauge length within a defined time, limited after-flame duration, and no ignition of filter paper by flaming droplets. The 50 W horizontal and vertical procedures of IEC 60695-11-10 and the UL 94 HB/V classifications are used for small molded plastic parts of the gangway hardware.
Lateral Flame Spread — EN ISO 5658-2
A vertically oriented specimen is exposed to a gas-fired radiant panel with pilot ignition. The test reports the critical heat flux at extinguishment (CFE) and the extent of lateral flame spread, which are key pass/fail parameters for interior lining requirement sets of EN 45545-2.
Heat Release — ISO 5660-1 / ASTM E1354 Cone Calorimeter
Specimens of 100 mm × 100 mm are irradiated at 25 kW/m² or 50 kW/m² under an exhaust hood with spark ignition. Peak heat release rate, total heat release over the first 600 s and MARHE are used within EN 45545-2 to quantify the contribution of bellows fabric and panels to fire growth.
Smoke Density — ISO 5659-2, ASTM E662, GB/T 8323, TB/T 3139
Specimens of 75 mm × 75 mm are mounted in a closed static chamber and exposed at 25 kW/m² with pilot flame and at 50 kW/m² without pilot flame, in both flaming and non-flaming modes. The resulting light-transmission curves yield Ds(1.5), Ds(4.0), Ds max and VOF4. NFPA 130 requires Ds(1.5) not exceeding 100 or Ds(4.0) not exceeding 200 for interior large-area components, while EN 45545-2 applies VOF4 and Ds(4)max limits that tighten from HL1 to HL3.
Smoke Toxicity — EN 17084, NF X 70-100-1/-2, GB/T 20285
Toxicity is assessed by analysing the gases evolved in the smoke chamber using FTIR spectroscopy or by a separate tube-furnace method. The conventional toxicity index CIT is calculated from the measured concentrations of CO, CO2, HCN, HCl, HF, HBr, SO2 and NOx against reference incapacitation concentrations. For Chinese metro projects, GB/T 20285 provides an additional danger-grading system for smoke produced by materials.
Flooring Radiant Panel — ISO 9239-1
Gangway floor coverings are tested as horizontal specimens of approximately 1050 mm × 230 mm inclined in a radiant gradient. The critical heat flux at extinguishment and the smoke development during the test demonstrate that the floor does not act as a path for fire spread between car bodies.
Flame Spread Index — ASTM E162
The radiant panel index test measures the ease of surface ignition and upward flame propagation on angled specimens. NFPA 130 applies a flame spread index limit of 25 or below to most interior components of transit vehicles.
Applications
Combustion test reports for coupling passage materials are used throughout the life cycle of metro rolling stock. Typical applications include:
- Type testing and homologation of new metro vehicles, including the compilation of EN 45545-2 classification evidence for all gangway components.
- Supplier qualification, first-article inspection and incoming material control at car builders and gangway manufacturers.
- Mid-life refurbishment and retrofit programs where original materials are replaced and re-qualified.
- Export projects that must satisfy the fire-safety framework of the destination market, such as EN 45545-2 in Europe or NFPA 130 in North America.
- Tender documentation, fire risk assessments and equivalence demonstrations for operators and design institutes.
- Development of flame-retardant rubber compounds, coated fabrics and intumescent coatings for next-generation lightweight gangways.
FAQ
Which fire standard governs metro coupling passage materials?
The governing framework depends on the market and the operator's specification. EN 45545-2 is the reference standard for European and many international projects; NFPA 130 applies in North America; TB/T 3138 and TB/T 3139 together with GB/T method standards apply to Chinese domestic rolling stock; and some legacy fleets still reference BS 6853, DIN 5510-2 or NF F 16-101. Accredited third-party laboratories can determine the applicable standard and requirement set for a specific project.
What hazard level applies to a metro gangway?
For automatically operated metro trains in long tunnels, EN 45545-1 normally assigns hazard level HL3, which imposes the strictest limits on flame spread, smoke density and toxicity. Manually operated suburban or elevated services may qualify for HL2 or HL1, and the vehicle builder confirms the level during design risk assessment.
What sample sizes and quantities are required?
Typical requirements are three to ten conditioned specimens per test: 75 mm × 75 mm for the smoke density chamber, 100 mm × 100 mm for the cone calorimeter, bars of roughly 80–150 mm × 10 mm for the oxygen index test, 90 mm × 190 mm for ISO 11925-2, 250 mm × 90 mm for flame spread tests, and full flooring panels of about 1050 mm × 230 mm for ISO 9239-1. Laboratories issue a sampling list once the requirement set is fixed.
Do the metal frames of the gangway need fire testing?
Bare metallic frames and plates are non-combustible and do not require combustion testing. However, the coatings, adhesives, gaskets, bushes and foams attached to or adjacent to them are combustible components and must be qualified individually or as part of the coated system.
What are typical acceptance criteria?
Commonly specified values include LOI of 28% or higher for bellows fabric, flame spread index not exceeding 25 per ASTM E162, smoke density limits such as Ds(1.5) ≤ 100 or Ds(4.0) ≤ 200 per NFPA 130, VOF4 and Ds(4)max limits per EN 45545-2 that tighten with hazard level, and a conventional toxicity index CIT below the category limit. The exact figures must always be read from the project specification and the applicable requirement set.
How long does a full test program take?
Conditioning to constant mass plus the full suite of flammability, smoke and toxicity tests typically requires four to eight weeks in an accredited third-party laboratory, depending on sample availability, the number of requirement sets and whether retests are needed after formulation adjustments.
