Railcars are classified in accordance with the fire hazard associated with their design and use. The three hazard levels are: HL1, HL2 and HL3, with HL1 setting the lowest requirements and HL3 the highest. The classification depends on a combination of two factors. The route that the train drives, with an important factor being how fast a passenger can reach a safe place in the event of a fire. An extreme example is the London Underground, where the tunnels are so narrow that escape is only possible at the front and back of the train. The second factor is the type of train, such as sleeper cars or double deckers, etc.