EN 45545-2 is the European standard for the fire behaviour of materials used on railway vehicles, and it grades vehicles into three hazard levels. HL1 is the least demanding and applies where passengers can leave the vehicle quickly and easily. HL3 is the most demanding and applies where evacuation is slow or difficult. HL2 sits between them.
The level is set for the vehicle, not the component, from its operation and design categories. It rises with the time a train spends in tunnels, with driverless operation, and for double-deck and sleeping-car designs. Every material on board, cables included, is then tested for properties such as flame spread, smoke density and smoke toxicity against the limits for that hazard level. A cable accepted for an HL2 vehicle is therefore not automatically acceptable for one specified to HL3, and the hazard level should be stated in any enquiry for rolling stock cables or harnesses.