Platform screen doors (PSDs), also known as platform edge doors (PEDs), are used at some train or subway stations to separate the platform from train tracks, as well as on some bus rapid transit systems. They are primarily used for passenger safety. They are a relatively new addition to many metro systems around the world, some having been retrofitted to established systems. They are widely used in Asian and European metro systems.
The idea for platform edge doors dates as early as 1908, when Charles S. Shute of Boston was granted a patent for "Safety fence and gate for railway-platforms". The invention consisted of "a fence for railway platform edges", composed of a series of pickets bolted to the platform edge, and vertically movable pickets that could retract into a platform edge when there was a train in the station. In 1917, Carl Albert West was granted a patent for "Gate for subrailways and the like". The invention provided for spaced guides secured to a tunnel's side wall, with "a gate having its ends guided in the guides, the ends and intermediate portions of the gate having rollers engaging the side wall". Pneumatic cylinders with pistons would be used to raise the gates above the platform when a train was in the station. Unlike Shute's invention, the entire platform gate was movable, and was to retract upward.
The first stations in the world with platform screen doors were the ten stations of the Saint Petersburg Metro's Line 2 that opened between 1961 and 1972. The platform "doors" are actually openings in the station wall, which supports the ceiling of the platform. The track tunnels adjoining the ten stations' island platforms were built with tunnel boring machines (TBMs), and the island platforms were actually located in a separate vault between the two track tunnels. Usually, TBMs bore the deep-level tunnels between stations, while the station vaults are dug out manually and contain both the tracks and the platform. However, in the case of the Saint Petersburg Metro, the TBMs bored a pair of continuous tunnels that passed through ten stations, and the stations themselves were built in vaults that only contained the platform, with small openings on the sides of the vault, in order for passengers to access the trains in the tunnels.
In 1987, Singapore's Mass Rapid Transit was the first metro system in the world to incorporate glass PSDs into its stations for climate control and safety reasons, rather than due to architectural constraints. All underground stations on all lines have these doors installed since their opening, and above-ground stations were retrofitted with the doors by the early 2010s. The design of the doors themselves differ depending on their installation location and time (see § Singapore). Hong Kong's MTR was the first metro system in the world to retrofit PSDs on a transit station already in operation.
The main reason platform screen doors are installed is for passenger safety. Their use helps prevent suicides, accidents, objects falling on the track and stops people entering tunnels. Platform screen doors also improve climate control within the station, which can lead to cost savings and reduced energy use by lowering the use of heating and air conditioning in the station. PSDs can also offer protection from the weather when used in open air.
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, 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, but they sometimes reach to the height of the train. These two types of platform screen doors are presently the main types in the world.
These doors help to:
Their primary disadvantage is their cost; installing a system typically costs several million USD per station. When used to retrofit older systems, they limit the kind of rolling stock that may be used on a line, as train doors must have exactly the same spacing as the platform doors; this results in additional costs due to depot upgrades and otherwise unnecessary purchases of rolling stock.
The doors also pose their own safety risks. The primary risk is that people may be trapped between the platform doors and the train carriage, and be subsequently crushed when the train begins to move (see § Incidents). Cases of this happening are rare, and may depend upon door design.
Half-height platform doors, or automatic platform gates, are chest-height sliding doors at the edge of railway platforms to prevent passengers from falling off the platform edge onto the railway tracks. Like full-height platform screen doors, these platform gates slide open or close simultaneously with the train doors.
Half-height platform gates are cheaper to install than platform screen doors, which require more metallic framework for support. Some railway operators may therefore prefer such an option to improve safety at railway platforms and, at the same time, keep costs low and non-air-conditioned platforms naturally ventilated. However, these gates are less effective than full platform screen doors in preventing people from intentionally jumping onto the tracks.
These gates were first in practical use by the Hong Kong metro system on the Disneyland Resort line for their open-air station design. The later design by other manufacturers, such as Manusa, Grupsa or Gilgen Door Systems AG, have their gates higher than the ones installed on the Disneyland Resort line.
There are also Rope type Platform Screen Doors, initially installed in some stations in South Korea and Japan, where multiple train types with different length and train door structures use the same platforms. The barriers move upwards, rather than sideways, when letting passengers through.
Sydney Metro Northwest, the initial section of the Sydney Metro, which opened in May 2019, was the first-fully automated rapid transit rail system in Australia. Full-height screen doors are provided on most underground platforms, with half-height doors on at-grade, elevated and some underground platforms. The existing five stations on the Epping to Chatswood railway line were upgraded to rapid transit standard, all being fitted with half-height platform screen doors.
In Melbourne, the Metro Tunnel, from South Kensington to South Yarra, due to open in 2025, will have platform screen doors on the underground stations. New rolling stock is being constructed, with doors that will line up with full-height ones on the platforms.
The Platform Screen Doors are present in the São Paulo Metro since 2010, when the Sacomã Station was opened. As of 2019, 5 of the 6 lines of the São Paulo Metro have the equipment: Line 4 - Yellow and Line 15 - Silver have the equipment installed in all of its stations. The feature is also present in some stations of Line 2 - Green, Line 3 - Red and Line 5 - Lilac. They are planned to be installed in 41 stations of lines 1, 2 and 3 by the end of 2021, as well as all stations of line 5 by the end of 2020.
PSDs are also found on Santos Light Rail since 2016.
In 2020, Rope type Screen Door (RSD) system was installed in Vasil Levski Stadium Metro Station and Opalchenska Metro Station of the Sofia Metro Line 1 and Line 2. In total, such rope type safety barriers will be installed on more 10 of the busiest stations on the Line 1 and 2 of the Sofia Metro, providing increased safety for passengers and protecting against accidental falls. 
Greater Montreal's forthcoming Réseau express métropolitain (REM), the 67-kilometre-long driverless complementary suburban rapid transit network opening in three phases between 2021 and 2023 will feature screen doors at each of its 26 stations.
With the advent of the REM on the horizon, advocating retrofits of platform edges in the Montreal Metro with doors to combat delays attributed to overcrowding is becoming increasingly customary. Were its type of door to be screen (full-height), then such installations might quash the fully underground system's notoriety whereby opening or passing through station entrance doors proves mightily troublesome due to the excessive windiness brought about by arriving or departing trains. Given the fact that there are two different door layouts on the Montreal Metro, with the older MR-73 trains having 4 doors on each side of the car, and MPM-10 having 3, it is unlikely platform doors will be showing up in the Montreal Metro in the near future.
All Chinese metro systems have platform screen doors installed on most of their lines. All stations built after the mid 2000s have some form of platform barrier. Only the Nanjing Metro, Dalian Metro and Wuhan Metro have stations without the platform screen doors on their early lines (As of 21 September 2019 ). However many are starting the process of retrofitting these lines with platform screen gates. In addition, many BRT systems such as the Guangzhou Bus Rapid Transit are also equipped with platform screen doors. Several underground high speed railway stations of the CRH network use platform screen doors set back from the platform edge.
Several stations on Bogota's TransMilenio bus rapid transit system use platform screen doors.
The Helsinki Metro had a trial run with Faiveley automatic platform gates installed on a single platform at Vuosaari metro station during phase one of the project. The doors, which are part of the Siemens metro automation project, were built in 2012. Phase 2 of the project has been delayed due to metro automation technical and safety related testings. The doors were removed in 2015.
All lines of the VAL automated subway system are equipped with platform screen doors at every station, starting with Lille subways in 1983. Those also include Toulouse and Rennes as well as the CDGVAL and Orlyval airport shuttles.
Paris Métro's line 14 from Saint-Lazare to Bibliothèque François Mitterrand was inaugurated in 1998 with platform screen doors manufactured by Faiveley Transport. The new station Olympiades opened with platform screen doors in June 2007. Line 1 has been retrofitted with platform edge doors, for full driverless automation effective in 2012. Some stations on Line 13 have had platform edge doors since 2010 to manage their overcrowding, after tests conducted in 2006, and stations on Line 4 are currently being fitted with platform edge doors in preparation for its automation.
Since 30 June 2020, a new kind of vertical platform screen doors, called platform curtains, are being tested on the platform 2bis of Vanves-Malakoff station (in Paris region) on the Transilien Line N commuter rail line. The experiment should end in February 2021. These 1.6 meter high vertical doors are manufactured by HENGZHU. Transilien said that they preferred platform curtains to classical screen doors for this line because the positioning of the doors is not the same across the rolling stock, and that they plan to install them in other Transilien stations if the experiment is successful.
The D line in Lyon, which is equipped with fully automated trains, does not have platform screen doors but identifies obstructions by infrared detectors upon the tracks. A similar system is employed by the metro system in Nuremberg, Germany.
People movers at Frankfurt International Airport, Munich International Airport and Düsseldorf Airport are equipped with platform screen doors, as well as the suspended monorail in Dortmund, called H-Bahn.
Today, all heavy rail platforms except those on the East Rail line have either platform screen doors or platform edge doors. Platform edge doors are only used on above-ground stations, while platform screen doors are used in both underground and above-ground stations. None of the light rail platforms has platform screen doors or platform edge doors.
The MTR Corporation had also, since mid-1996, been studying the feasibility of installing PSDs at the older stations. It decided in 1999 to undertake the PSD Retrofitting Programme at 74 platforms of 30 underground stations on the Kwun Tong, Island, and Tsuen Wan lines. This was completed in early 2006. The Mass Transit Railway was the first metro system in the world to retrofit PSDs on a transit system already in operation.
In 2006 the MTR began studying ways to introduce barriers at above-ground and at-grade stations, which was considered more complicated as these stations are naturally ventilated and the introduction of platform screen doors would entail the installation of air conditioning systems. In 2008 the corporation decided to install automatic platform gates at these eight stations (the MTR and KCR Corporation were operationally merged in 2007, but KCR stations were except from this study). The eight stations were retrofitted with APGs in 2011.
From July 2000 to December 2013, the MTR Corporation collected a surcharge of 10 cents from each Octopus-paying passenger to help pay for the installation of PSDs and APGs. Over HK$1.15 billion was collected in total.
Platform screen doors were also installed on all platforms of the Tuen Ma line, then built by the Kowloon-Canton Railway Corporation (KCRC) before the MTR-KCR merger. The Ma On Shan line did not have gates upon opening even though it was built at the same time as the West Rail.
The installation of platform screen doors in Hong Kong has been effective in reducing railway injuries and service disruptions.
The longest set of platform screen doors in the world can be found in East Tsim Sha Tsui station. A reduction of train length from 12 to 7 cars following the construction of Kowloon Southern Link caused many of the screen doors to be put out of service, although the trains were lengthened to eight cars in May 2018.
The platform screen doors presently in service in the MTR were supplied by the Swiss manufacturer Kaba Gilgen, the Japanese Nabtesco Corporation (under the Nabco brand), as well as Faiveley Transport. The last stations in Hong Kong without platform screen doors or gates are all found on the East Rail line and Ma On Shan line, both former KCR lines not part of the MTR APG retrofitting programmes. However, these remaining stations are all being retrofitted by Kaba as part of the Sha Tin to Central Link project. Adding APGs to the East Rail platforms requires platform strengthening as the gates, combined with heavy winds, can greatly increase structural loading on the platform edge.
Apart from the MTR, all stations on the Hong Kong International Airport Automated People Mover are equipped with platform screen doors made from Westinghouse Platform Screen Doors (for Phase 1) and Panasonic (for Midfield Extension).
On the Delhi Metro, all stations on the Delhi Airport Metro Express line, which links to Indira Gandhi International Airport have been equipped with full-height platform screen doors since November 2015 and the six busiest stations on the Yellow Line have also been equipped with half height platform gates. Automatic platform gates on all the stations of the Pink and Magenta Line.
There are platform screen doors in all elevated stations of Kolkata Metro Line 2. Platform screen doors will be introduced in underground stations of Kolkata Metro Line 2, Kolkata Metro Line 3, Kolkata Metro Line 6. There are plans to install platform screen doors also in Kolkata Metro Line 1.
The Pune Metro's purple and aqua lines will have half-height PSDs at their elevated stations and full height ones at underground stations when the open in early 2022.
On the Mumbai Metro, all lines being made by MMRDA will have half-height platform screen doors on all elevated stations and full-height platform screen doors in the underground stations, as the trains used in these lines have a GoA level 4, and also to reduce risk of passenger deaths by overcrowding. In Line 3, the Aqua line, will have full-height platform screen doors, as the line is fully underground, and like the MMRDA lines above, will have GoA level 4 (Unattended train operation).   
Platform screen doors are in use on the underground stations on the new Jakarta MRT and half-height doors are used on above-ground stations. Soekarno-Hatta Airport Rail Link stations, such as SHIA railway station, also uses platform screen doors. The Soekarno-Hatta Airport Skytrain are also equipped with platform screen doors. The Jakarta LRT and future Greater Jakarta LRT also uses half-height doors at its stations (all above ground). The TransJakarta bus rapid transit system also uses automatic half-height doors at the Bundaran HI and Tosari stations. In the future, platform screen doors and half height doors will be installed on Phase 2 and 3 of the Jakarta MRT.
Platform screen doors at one of SHIA Skytrain station
Platform screen doors at SHIA railway station
Platform screen doors at Bundaran HI MRT station
Platform screen doors are used in most newly built rapid transit lines and systems of new construction in Italy. PSDs are present on Turin Metro, the Venice People Mover, the Perugia Minimetrò, the Brescia Metro, Line 5 of the Milan Metro, and Line C of the Rome Metro.
The Tokyo Metro and Toei Subway began using barriers with the 1991 opening of the Namboku Line (which has full-height platform screen doors), and subsequently installed automatic platform gates on the Mita, Marunouchi, and Fukutoshin lines. Some railway lines, including the subway systems in Sapporo, Sendai, Nagoya, Osaka, Kyoto, and Fukuoka, also utilize barriers to some extent.
In August 2012, the Japanese government announced plans to install barriers at stations used by 100,000 or more people per day, and the Ministry of Land, Infrastructure, Transport and Tourism allotted 36 million yen ($470,800) for research and development of the system the 2011-2012 fiscal year. A difficulty was the fact that some stations are used by different types of trains with different designs, making barrier design a challenge.
In 2018, automatic platform gates will be installed on the S?bu Rapid Line platforms at Shin-Koiwa. As these trains are 300 m (980 ft) long, it is believed that this will break the world record for the longest platform doors at East Tsim Sha Tsui station in Hong Kong.[needs update]
Platform screen doors (PSD) are installed at all underground Kelana Jaya Line stations, from Ampang Park to Masjid Jamek and Kajang Line, from Muzium Negara to Maluri stations. The automated announcement message reading "For safety reasons, please stand behind the yellow line" in both English and Malay languages are also heard before the train arrived at all stations.
PSD at KLCC station
PSD at Tun Razak Exchange station
PSD at KLIA Satellite A Building
PSG at Semantan station
In Pakistan, platform screen doors are installed at all stations of bus rapid transits Lahore Metrobus and Rawalpindi-Islamabad Metrobus, as well as all stations of the Lahore Metro, with half-height doors for elevated stations, and full-height doors for underground stations. There are plans to install them at all stations of the Multan Metrobus and the Karachi Metrobus.
In Peru, several stations of the Metropolitano bus rapid transit system in Lima use platform screen doors. The platform screen doors will be installed on Line 2 of the Lima Metro (under construction).
Half-height platform screen doors shall be installed on the North-South Commuter Railway, while full-height platform screen doors shall be installed on the Metro Manila Subway. The system is sought to open in stages between 2022 and 2025.
Park Pobedy (Russian ) is a station of the Saint Petersburg Metro that was the first station in the world with platform doors. The station was opened in 1961. Later, nine more stations of this type were built in Leningrad (nowadays Saint Petersburg): Petrogradskaya (Russian), Vasileostrovskaya (Russian), Gostiny Dvor (Russian? ?), Mayakovskaya (Russian), Ploshchad Alexandra Nevskogo I (Russian ? -1), Moskovskaya (Russian), Yelizarovskaya (Russian: ), Lomonosovskaya (Russian), and Zvyozdnaya (Russian?).
There was an electronic device to ensure that the train stopped with its doors adjacent to the platform doors; they were installed so that driverless trains could eventually be used on the line. Unlike other platform screen doors, which are lightweight units with extensive glazing installed on a normal platform edge, the St Petersburg units give the appearance of a solid wall with heavyweight doorways and solid steel sliding doors, similar to a bank of elevators in a large building, and the train cannot be seen entering from the platform; passengers become familiar with the sound alone to indicate a train arrival.
In May 2018, two other similar stations were opened: Novokrestovskaya and Begovaya. Unlike the first ten stations that were built, these stations utilize glass screen doors, allowing the train to be seen from entering the platform.
The Singapore Mass Rapid Transit (MRT) was the first rapid transit system in Asia to incorporate platform screen doors in its stations in 1987. Additionally, the Singapore Mass Rapid Transit (MRT) was the first rail system in the world to incorporate platform screen doors consistently at all stations on the network with half height variants on elevated stations. Full height PSDs mainly manufactured by Westinghouse are installed at all existing underground MRT and sub-surface stations in Singapore, as well as at Ten Mile Junction LRT station since its opening in 1999, but which has since been permanently closed. Future underground MRT stations will also have full-height doors installed upon opening. Half-height platform screen doors mostly manufactured by ST Electronics have been retrofitted into all elevated stations by March 2012 (starting with three elevated MRT stations in 2009), as well as all future above-ground MRT stations. The LRT stations at Sengkang, Bukit Panjang and Punggol also have half height platform screen panels installed beginning in 2015. These panels lack physical doors and vary in size according to their location on the platform. All remaining LRT stations received these panels by 2018.
Platform screen doors were introduced primarily to minimise hefty air-conditioning costs, especially since elevated stations are not air-conditioned and are much more economical to run in comparison. Since then the safety aspects of these doors have become more important, as highlighted by a series of high-profile incidents where individuals were injured or killed by oncoming trains since the year 2002--all occurring on elevated stations with no screen doors. The Land Transport Authority reports that there have been more than 220 cases of commuters trespassing on the tracks between 1991 and 15 September 2004, of which 87 percent were deliberate acts of trespass. Nine fatalities were recorded during this period, of which only one was an accident. Since September 2004 there have been six fatalities occurring on elevated MRT and LRT stations.
There are two series of the full-height platform screen doors in use. The first series, made by Westinghouse, was installed at all underground stations along the North South line and the East West line from 1987 to the completion of the initial system in 1990. The second series of PSDs sport a sleeker design, producing less sound when the doors open and close while incorporating more glass. The first station with these doors was Changi Airport MRT station which opened in 2002. All stations in the fully underground North East line and Circle line, which opened in 2003 and 2010 respectively, sport these new doors, also made by Westinghouse, although there are slight differences in their designs. The same doors used in the Circle line also equip the rebuilt North South line platforms at Bishan MRT station in 2009 following renovations in conjunction with the Circle line interchange at the station. The third series of PSDs sport an even sleeker design, starting from the Downtown line which also features Westinghouse doors. Also, they are found in Marina South Pier MRT station which opened in 2014. The Thomson-East Coast line uses full-height doors by a different manufacturer, GE Transportation, in its stations. Platform screen doors are also present on the Changi Airport Skytrain and the Sentosa Express.
Half-height platform doors were installed at all elevated train stations in Singapore, such as Jurong East station
Yongdu station of Seoul Subway Line 2 was the first station on the Seoul Subway to feature platform screen doors; the station opened in October 2005. By the end of 2009, many of the 289 stations operated by Seoul Metro had platform doors by Hyundai Elevator. Seoul Metro Line 1, 2, 3, 4, 5, 6, 7, 8 and 9 were equipped with platform screen doors. However, not all stations operated by Korail have completed installation. All stations in South Korea will have platform screen doors by 2018. As of 2017, 100% of subway stations are equipped with platform screen doors in Daejeon, Gwangju, Busan, Incheon and Daegu.
The platform screen doors, installed in Munyang station in Daegu Metro Line 2 by The Korea Transport Institute in 2013, have a unique rope-based platform screen named Rope type Platform Safety Door (RPSD). A door sets of rope blocks separate the platform from the rails. When the train arrives, the rope screen door sets are vertically opened and allow passenger boarding to and from the train. This RPSD was also used in Nokdong station on Gwangju Metro Line 1, but was removed in 2012, and a new full-height platform screen door was installed in 2016 instead.
Half platform screens were installed first in Provença FGC station (Barcelona) around 2003. Later doors were tested on Barcelona Metro line 11 before fitting them on all stations for the new lines 9 and 10, which operate driverless. In the Seville Metro this system is the one used since it was opened in April 2009.
Stockholm commuter rail has platform doors on two underground stations opened in July 2017, as part of the Stockholm City Line. The Stockholm metro will test platform doors at Bagarmossen metro station in 2021. The underground Liseberg station in Gothenburg has platform doors which were built before its opening 1993. The reason was safety against the freight trains that go in this tunnel. These doors are built one meter from the platform edge and do not restrict the train type.
Zurich International Airport's Skymetro shuttle between the main building (hosting terminals A and B) and the detached terminal E has glass screen doors separating the tracks from the passenger hall platforms at both ends.
Lausanne Metro's Line M2 has glass screen doors at every station.
On Taipei Metro, platform screen doors were first installed on the Wenhu line (then known as Muzha line) in 1996. Older high-capacity MRT lines (Tamsui-Xinyi Line, Songshan-Xindian Line, Zhonghe-Xinlu line, and the Bannan Line) were initially constructed without platform screen doors but are retrofitted with automatic platform gates which is completed in 2018. Newer stations, on the Xinyi Line(part of the Tamsui-Xinyi Line), Luzhou and Xinzhuang Line (part of the Zhonghe-Xinlu Line), Songshan Line (part of the Songshan-Xindian Line), Circular Line, and part of the Bannan Line's Dingpu Station and Taipei Nangang Exhibition Center Station) are constructed with platform screen doors.
The Taoyuan Metro Airport Line is equipped with platform screen doors on underground stations and automatic platform gates in elevated stations.
In the Taoyuan Int. Airport Skytrain, platform screen doors are installed on all Stations (Terminal 1 and Terminal 2, and possibly Terminal 3)
On the Kaohsiung Metro, the underground section of the Metro are equipped with platform screen doors.
Platform Screen Doors were first installed on the Bangkok MRT System since it first operates. Followed by the Airport Rail Link System in Makkasan Station (Express Platform) and Suvarnabhumi Station (both City and Express Line Platform. BTS Skytrain system first installed the Platform screen doors at Siam Station. Followed by all the busy stations during rush hours. Today, Almost all stations on the Bangkok Electrified Rail System have installed the Platform Screen Doors preventing people from falling onto the tracks. BTS Skytrain system has installed PSDs on 18 stations out of its 44 stations. Both Purple and Blue Line of Bangkok MRT system has installed PSDs on all of its stations. Airport Rail link has installed a stainless steel fence to prevent from falling but has not installed an automated door due to a speed of the train is too fast that it will break the glass. All new stations in Bangkok must install Platform screen doors as a rule.
Platform screen doors are installed on all the platforms in the fully automated Dubai Metro, as well as on the Dubai Airport People Mover, Palm Jumeirah Monorail and Dubai Tram (the world's first tram system to feature platform screen doors).
The Jubilee Line Extension project saw platform edge doors installed on its new stations that were underground, and were produced by Westinghouse. There are plans to install PEDs (which, similar to the Jubilee Line, will not reach to the ceiling) in existing London Underground stations along the Bakerloo, Central, Piccadilly, and Waterloo & City lines as part of New Tube for London.
PEDs are present on the Gatwick Airport shuttle system, Heathrow Airport Terminal 5 airside people mover shuttle, Birmingham Airport AirRail Link and Stansted Airport Transit System. In these cases the purpose is safety, as there is a considerable drop on the other side of the doors.
The Elizabeth Line, the new cross-city line for London (delivered as the Crossrail Project) has Platform Screen Doors on each of the sixteen sub-surface platforms of its central section. Each platform has twenty-seven doors which align with the twenty-seven saloon doors of the new British Rail Class 345 which will operate the service. The doors form a 2.5m high glass and steel screen the entire length of the platform. The system is unusual in that the trains served are full-sized commuter trains, larger and longer than the trains of metro systems more commonly equipped with Platform Screen Doors. In total, some 4km of platform screen is provided.
Platform screen doors are rare in the United States, and with one exception, is exclusively found on small-scale systems. The Las Vegas Monorail system is currently the only general-purpose rapid transit system in the US to use platform screen doors. Honolulu Rail Transit will become the first large-scale publicly run metro system in the United States to feature platform screen doors, when it opens in late 2021.
New York City's Metropolitan Transportation Authority has not committed to installing platform screen doors in its subway system, though it had been considering such an idea since the 1980s. Possible locations for platform doors include several stations along the Second Avenue Subway, but their installation presents substantial technical challenges, as there are different placements of doors on New York City Subway rolling stock. The MTA is also interested in retrofitting platform screen doors on the Canarsie Line, along the train, and on the IRT Flushing Line, along the and trains. However, it is unlikely that the entire New York City subway system will get retrofitted with platform screen doors or automatic platform gates due to, again, the varying placements of doors on rolling stock. Following a series of incidents during one week in November 2016, in which three people were injured or killed after being pushed into tracks, the MTA started to consider installing platform edge doors for the 42nd Street Shuttle. In October 2017, the MTA formally announced that platform screen doors would be installed at the Third Avenue station on the train as part of a pilot program, but the pilot was later postponed after funding for the doors was reallocated to an elevator installation project.
As for other systems, it is most frequently a lack of interest (or potentially thought) that explains the absence of platform doors, even when they theoretically could be installed.
People movers, systems that ferry passengers across large distances they would otherwise walk, make use of platform screen doors. These systems are common at airports such as Hartsfield-Jackson Atlanta International Airport and Denver International Airport. The Port Authority of New York and New Jersey uses full height platform screen doors at two of its systems: AirTrain JFK and AirTrain Newark (serving John F. Kennedy International Airport and Newark Liberty International Airport respectively). San Francisco International Airport has AirTrain, a 6-mile-long line whose stations are fully enclosed with platform screen doors, allowing access to the fully automated people mover. Chicago O'Hare International Airport has a people mover system which operates 24 hours a day and is a 2.5 mile long (4 km) line that operates between the four terminals at the airport and parking areas; each station is fully enclosed with platform screen doors allowing access to the fully automated people mover trains. AeroTrain is a 3.78-mile (6.08 km) people mover system at Washington Dulles International Airport in Dulles, Virginia with fully enclosed tracks including platform screen doors. The United States Capitol subway system, a train cart people mover system, uses platform gates.
On the Shanghai Metro in 2007, a man forcing his way onto a crowded train became trapped between the train door and platform door as they closed. He was pulled under the departing train and killed. An almost identical death occurred on the Beijing Subway in 2014?—?the third death involving platform doors in China within the several years preceding it. In 2018, a woman was similarly trapped between the platform doors and train at Shanghai's Bao'an Highway station. She escaped injury by standing still as the train departed.