platform edge doors. On the East Rail line, PSDs are installed only at , and stations. Platform edge doors are currently only used in at-grade and elevated stations, while platform screen doors are used in all underground and some at-grade or elevated stations. None of the light rail platforms have platform screen doors or platform edge doors installed. The MTR Corporation had since mid-1996, been studying the feasibility of installing PSDs at the older stations to reduce suicides on the MTR and reduce air-conditioning costs. Platforms 2 and 3 of were chosen for the trial due to them being redundant platforms and receiving low numbers of passengers. Platform screen doors of two and a half cars' length were installed on each of the two platforms during the trial in 1996. As the Kwun Tong line trains consisted of 8 cars, it was decided that the PSDs were to be removed to allow for smoother train operations. With the opening of the and , Hong Kong had its first full-height PSDs fully operational in 1998. The MTR decided in 1999 to undertake the PSD Retrofitting Programme at 74 platforms of 30 select underground stations on the Kwun Tong, Island, and s. 2,960 pairs of PSDs were ordered from Gilgen Door Systems. Choi Hung became the first station to receive platform screen doors from this programme in August 2001. The Mass Transit Railway became the first metro system in the world to retrofit PSDs on a transit system already in operation. The program was completed in March 2006. All subsequent new stations or platforms installed with PSDs also used those manufctured by Gilgen Door Systems. The opening of the and stations in 2005 also meant the first platform-edge doors entering operation for the MTR network. These doors are currently the lowest in the entire network of being at around high, compared to on the Kwun Tong, Tsuen Wan, Island and Tung Chung lines and on the Tuen Ma and s. In 2006, the MTR began studying ways to introduce barriers at above-ground and at-grade stations, which was considered more complicated as those stations were naturally ventilated and the introduction of full-height platform screen doors would entail the installation of air conditioning systems. In 2008, the corporation decided to install automatic platform gates (APGs) at eight stations (the MTR Corporation Limited and KCR Corporation had been operationally merged since 2007, but KCR stations