opened in 1987 is often described as the first heavy metro system in the world to incorporate PSDs into its stations for climate control and safety reasons, rather than due to architectural constraints, though the light Lille Metro opened in 1983 predates it. Types Although the terms are often used interchangeably, platform screen doors can refer to both full-height and half-height barriers. Full height platform screen doors are total barriers between the station floor and ceiling, while the half-height platform screen doors are referred to as platform edge doors or automatic platform gates, as they do not reach the ceiling and thus do not create a total barrier. Platform gates are usually only half of the height of the full-screen doors, are chest-height sliding doors at the edge of railway platforms to prevent passengers from falling off the platform edge onto the railway tracks. But they sometimes reach to the height of the train. Like full-height platform screen doors, these platform gates slide open or close simultaneously with the train doors. These two types of platform screen doors are presently the main types in the world. Platform screen doors and platform edge doors These doors help to: Prevent people from accidentally falling onto the tracks, getting too close to moving trains, and committing suicide (by jumping) or homicide (by pushing). Prevent or reduce wind felt by the passengers caused by the piston effect which could in some circumstances make people lose their balance. Improve safety—reduce the risk of accidents, especially from trains passing through the station at high speeds. Improve climate control within the station (heating, ventilation, and air conditioning are more effective when the station is physically isolated from the tunnel). Improve security—access to the tracks and tunnels is restricted. Lower costs—eliminate the need for motormen or conductors when used in conjunction with automatic train operation, thereby reducing manpower costs. Prevent litter buildup on the tracks, which can be a fire risk, as well as damage and possibly obstruct trains. Improve the sound quality of platform announcements, as background noise from the tunnels and trains that are entering or exiting is reduced. At underground or indoor platforms, prevent the air from being polluted by the fumes caused by friction from the train wheels grinding against the tracks. Their primary disadvantage is their cost; installing a system typically costs several million USD per station. When used to retrofit older systems,