
Selective door operation (SDO), also known as selective door opening, is a mechanism employed primarily on
train
A train (from Old French , from Latin">-4; we might wonder whether there's a point at which it's appropriate to talk of the beginnings of French, that is, when it wa ... , from Latin , "to pull, to draw") is a series of connected vehicles th ...
s (although
buses with multiple doors also generally have this feature) that allows the driver or conductor/guard to open the doors of a train separately.
Selective Door Operation enables trains to call at a station where the platform is shorter than the train. Some doors can be prevented from opening to ensure that passengers do not disembark from any carriages not standing at the platform. The term Selective Door Operation is used mainly in the
United Kingdom
The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...
; some
train operating companies
In the railway system of Great Britain, a train operating company (TOC) is a railway undertaking operating passenger trains under the collective National Rail brand. TOCs have existed since the privatisation of the network under the Railways ...
used the term ‘Door De-Select’. A version of this is used in other countries and on other rail systems such as the
London Underground
The London Underground (also known simply as the Underground or as the Tube) is a rapid transit system serving Greater London and some parts of the adjacent home counties of Buckinghamshire, Essex and Hertfordshire in England.
The Undergro ...
.
UK variations

In the UK various trains, either
multiple unit
A multiple-unit train (or multiple unit (MU)) is a self-propelled train composed of one or more Coach (rail), carriages joined, and where one or more of the carriages have the means of propulsion built in. By contrast, a locomotive-hauled ...
s or
coaches, have variations of the selective door operating system. This usually depends on what the specific
train operating company
In the railway system of Great Britain, a train operating company (TOC) is a railway undertaking operating passenger trains under the collective National Rail brand. TOCs have existed since the privatisation of the network under the Railways ...
and/or
train leasing company required, either at time of purchase or a later modification to an existing train to keep up to date with regulations. Examples of these variations are as follows:
*Most
Class 170 Turbostar units, with certain exceptions like the 170 Mark 1, are fitted with SDO. This when operated de-selects all carriages behind the carriage in which the train doors are being operated, so the guard can operate the SDO, allowing any length of train to occupy the platform as long as it can take one carriage.
*Former
Midland Mainline Class 170 'Mark 1' Turbostar units are fitted with SDO. This form of SDO was operated by the driver, who would run the leading carriage off the platform and deactivate the doors in the leading cab before releasing. These units are currently in use with
CrossCountry
CrossCountry (legal name XC Trains Limited) is a British train operating company owned by Arriva UK Trains, operating the current CrossCountry franchise.
The CrossCountry franchise was restructured by the Department for Transport (DfT) in 2006, ...
, without any need for this form of SDO.
*
Class 172 units have an SDO feature fitted to them, working in a similar same way to the Class 170. The system on 172 defaults to SDO (unlike the 170) and requires the Guard to deactivate before releasing all the doors. This includes the units
London Overground
London Overground (also known simply as the Overground) is a Urban rail in the United Kingdom, suburban rail network serving London and its environs. Established in 2007 to take over Silverlink Metro routes, it now serves a large part of Greate ...
formerly operated on the Gospel Oak Branch, although
Driver Only is now in operation on the entire London Overground network.
*
Class 350 Desiro electric multiple units use a system call Unit De-Select. This allows the guard of the train to de-select an entire unit on a train while they are in working in multiple from one of the driving cabs, meaning that an 8 coach 350 set for example, made up of two units (each unit has 4 coaches), can have one set de-selected.
*Since 2015, Class
444 and
450 Desiro electric multiple units have used a system called Automatic Selective Door Operation. As the train approaches a platform, a beacon between the rails communicates with the train's on board computer, which tells it the number of coaches to release doors on. If for any reason the computer does not receive a signal, it will release only the front 4 coaches - the length of the shortest platform served by
South Western Railway
South Western Railway Limited, trading as South Western Railway (SWR), is the British state-owned train operating company that took over the services of the South Western Railway (2017–2025), operator of the same name from FirstGroup and MTR ...
, the operator of the Class 444 and 450s.
*
Great Western Railway
The Great Western Railway (GWR) was a History of rail transport in Great Britain, British railway company that linked London with the southwest, west and West Midlands (region), West Midlands of England and most of Wales. It was founded in 1833, ...
High Speed Train
High-speed rail (HSR) is a type of rail transport network utilising trains that run significantly faster than those of traditional rail, using an integrated system of specialised rolling stock and dedicated railway track, tracks. While there is ...
sets have SDO at almost all door locations. These trains are of the slam-door variety and fitted with the central door locking system. The guard operates the SDO system from most door control panels throughout the train. The guard can then either de-select doors in front of that location or behind.
*
Class 180 units are fitted with SDO, this is operated by the driver (who also releases the doors) from a switch in the leading cab.
* Most Electrostars (Bombardier built)
Class 375,
377,
378,
379 and
387
__NOTOC__
Year 387 (Roman numerals, CCCLXXXVII) was a common year starting on Friday of the Julian calendar. At the time, it was known in Rome as the Year of the Consulship of Valentinian II, Augustus and Eutropius (historian), Eutropius (or, l ...
have SDO systems fitted and operate in the southeastern region of the country. A
Global Positioning System
The Global Positioning System (GPS) is a satellite-based hyperbolic navigation system owned by the United States Space Force and operated by Mission Delta 31. It is one of the global navigation satellite systems (GNSS) that provide ge ...
(GPS) receiver located on the train passes location data to the MITRAC Train Control Management System,
which uses its database of platform lengths to determine how many doors will be released when the driver presses the 'door open' buttons at a station. The Class 387s in use on the
Govia Thameslink routes have an additional Tracklink II
system to augment the GPS. The Tracklink II system consists of a
balise fitted at short platform stations which sends data to the approaching train showing which station it is approaching, and the length of the platform that it is entering. The Tracklink II system is necessary because GPS is not accurate enough to differentiate between platforms at a multi-platform station.
* All
Class 195 and
Class 331 CAF Civity trains have ASDO systems fitted, the vast majority of services will operate as 3 and 4-car formations (doubled 2-car for the Class 195), some 3-car units will be doubled for 6-car services. ASDO is fitted on routes where platform lengths cannot fully accommodate the train. The ASDO system is linked to an automated system which informs passengers through announcements and the passenger information screens located in each saloon.
*On
Class 373 trains, manually locked selective door opening was used at shorter platforms when sets ran with
GNER on their "White Rose" services between
London King's Cross,
York
York is a cathedral city in North Yorkshire, England, with Roman Britain, Roman origins, sited at the confluence of the rivers River Ouse, Yorkshire, Ouse and River Foss, Foss. It has many historic buildings and other structures, such as a Yor ...
and
Leeds
Leeds is a city in West Yorkshire, England. It is the largest settlement in Yorkshire and the administrative centre of the City of Leeds Metropolitan Borough, which is the second most populous district in the United Kingdom. It is built aro ...
.
International variations
Selective door operation is implemented at certain railway stations in the United States. In the
New York City Subway
The New York City Subway is a rapid transit system in New York City serving the New York City boroughs, boroughs of Manhattan, Brooklyn, Queens, and the Bronx. It is owned by the government of New York City and leased to the New York City Tr ...
, the -car-long platforms at
145th Street (and formerly the 5-car-long loop platforms at
South Ferry) are too short to accommodate full-length trains of ten cars, so only the first five cars of the train opened their doors at these stations. Also at South Ferry, the inner platform's curves were so tight that only the inner doors of the cars were able to be opened. At
Times Square
Times Square is a major commercial intersection, tourist destination, entertainment hub, and Neighborhoods in New York City, neighborhood in the Midtown Manhattan section of New York City. It is formed by the junction of Broadway (Manhattan), ...
on track 4 of the
42nd Street Shuttle
The 42nd Street Shuttle is a New York City Subway shuttle train service that operates in Manhattan. The shuttle is sometimes referred to as the Grand Central/Times Square Shuttle, since these are the only two stations it serves. The shuttle op ...
, the original platform was so short that only the first door of the third car was allowed to open at the station. After a reconfiguration of the shuttle in 2021, it is now possible for an entire train to fit on track 4.
On the
Staten Island Railway
The Staten Island Railway (SIR) is a rapid transit, railroad line in the New York City borough (New York City), borough of Staten Island. It is owned by the Staten Island Rapid Transit Operating Authority (SIRTOA), a subsidiary of the Metropol ...
, the
Richmond Valley station only allows for the first three cars to open, due to the short platforms which cannot fit four cars. In addition, the last car for St. George-bound trains do not open at
Clifton, due to the wide space from trains caused by the platform's curvature. Before they were demolished in 2017, the
Atlantic
The Atlantic Ocean is the second largest of the world's five oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for se ...
and
Nassau stations only allowed the last car of a train to open its doors at the platforms.
In
Boston
Boston is the capital and most populous city in the Commonwealth (U.S. state), Commonwealth of Massachusetts in the United States. The city serves as the cultural and Financial centre, financial center of New England, a region of the Northeas ...
, the boarding platform at the
Bowdoin terminus of the
MBTA
The Massachusetts Bay Transportation Authority (abbreviated MBTA and known colloquially as "the T") is the public agency responsible for operating most public transportation services in Greater Boston, Massachusetts. The MBTA transit network in ...
Blue Line accommodates only four of the train's six cars; passengers must press buttons to open the doors. Similar selective door operation protocols are used on many commuter rail lines within the
Northeast megalopolis
The Northeast megalopolis, also known as the Northeast Corridor, Acela Corridor, Boston–Washington corridor, BosWash, or BosNYWash, is the most populous megalopolis exclusively within the United States, with slightly over 50 million resident ...
since some commuter rail stations have platforms that are too short to accommodate longer trains.
In
Seattle
Seattle ( ) is the most populous city in the U.S. state of Washington and in the Pacific Northwest region of North America. With a population of 780,995 in 2024, it is the 18th-most populous city in the United States. The city is the cou ...
, the
Satellite Transit System at
Seattle-Tacoma International Airport uses selective door operation on the loop connecting the South Satellite (the airport's international concourse) with the airport's main terminal. Passengers on most international flights arrive at the South Satellite, where they are inspected by
U.S. Customs and Border Protection
United States Customs and Border Protection (CBP) is the largest federal law enforcement agency of the United States Department of Homeland Security. It is the country's primary border control organization, charged with regulating and facilita ...
officials; after clearing inspection, passengers have the option of waiting in line to be inspected by the
Transportation Security Administration
The Transportation Security Administration (TSA) is an agency of the United States Department of Homeland Security (DHS) that has authority over the security of transportation systems within and connecting to the United States. It was created ...
so that they may access the rear two cars and secure area of the airport (to catch a connecting flight) or directly boarding the first train car which transports them to the airport exit and the baggage claim area. The platform screen doors that provide access to the first car of the train do not open at the station serving Concourse B of the airport to prevent unscreened passengers from accessing the concourse.
In many
NSW TrainLink
NSW TrainLink is a regional train and coach operator in Australia, providing services throughout New South Wales and into Australian Capital Territory, the Australian Capital Territory, Victoria (state), Victoria, Queensland and South Australia ...
stations in New South Wales, Australia, due to shorter platform length than trains, only selected doors will open. One example would be
Wondabyne railway station or
Zig Zag railway station, where the extremely short length of platform only allows rear door (next to crew cab) on the last carriage to be opened.
In New Zealand, “selective door opening” is used on the
Wairarapa Connection
The Wairarapa Line is a New Zealand interurban commuter rail service along the Wairarapa Line between Masterton, the largest town in the Wairarapa, and Wellington. It is operated by Wellington suburban operator Transdev Wellington, Transdev (w ...
commuter train, as the
Maymorn railway station platform is not long enough to accommodate all the carriages, and Maymorn passengers are restricted to the first three carriages.
Limitations
Most modern Selective Door Opening (SDO) systems receive their positioning data from the
Global Positioning System
The Global Positioning System (GPS) is a satellite-based hyperbolic navigation system owned by the United States Space Force and operated by Mission Delta 31. It is one of the global navigation satellite systems (GNSS) that provide ge ...
(GPS). As the train arrives at a station, the GPS
determines the location for the train's SDO control, which contains a database with a Unique Location Identifier (ULI) for each station. This then enables the correct number of coaches to be opened to suit the length of the platform. However this system is reliant on the train stopping in the correct position, since the SDO will authorise the doors to open as long as the train is within 300 metres of the station.
Depending on which system is in use, SDO may not prevent the doors from being opened where there is no platform if;
* The train stops beyond the appropriate stop mark and the driver releases the doors
* The train stops before the appropriate stop mark and the driver releases the doors
* The driver releases the doors on the wrong side of the train
Local door operation
Selective Door Operation is different from Local Door Operation (LDO), which is used on many trains by train crew and other staff.
References
* https://www.railsigns.uk/sect21page4.html
* http://www.hima-sella.co.uk/cgi-bin/scribe?showinfo=Tracklink-Selective-Door-Opening-SDO-Hima-Sella;from=pg03
Hima-Sella Ltd Tracklink II system
{{DEFAULTSORT:Selective Door Operation
Rail technologies
Passenger rail transport in the United Kingdom