Tung Chung Line, the Airport Express and Tseung Kwan O Line, except Quarry Bay station, had platform screen doors (PSDs) installed upon construction. These doors prevent people from falling onto the rails, and MTRC allowed the assumption that this implementation was primarily a safety campaign, without heavily promoting it directly. In fact, the primary motivation was to separate the stations from the tunnels, hence allowing substantial energy savings on station air-conditioning and tunnel ventilation. Automatic platform gates (APGs) have also been installed at the Sunny Bay and Disneyland Resort stations. Their heights are half of the PSDs and only prevent people from falling onto the rails.