Selective Door Operation

Selective Door Operation, also called Selective Door Opening (or SDO) is a mechanism employed primarily on trains (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 (SDO) is a system used by trains for the safe operation of the passenger doors at short station platforms. The term Selective Door Operation is used mainly in the United Kingdom; some train operating companies 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.

To simplify, some trains that call at certain railway stations are too long for the platform. This caused an operational headache on old stock but was solved by Selective Door Operation. The guard or driver can choose which doors are to be opened so as not to allow passengers to disembark from carriages not standing at the platform.

In the UK various trains, either multiple units or coaches, have variations of the Selective Door Operating system. This usually depends on what the specific train operating company and/or 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:

Selective door operation is implemented at certain railway stations in the United States. In the New York City Subway, the platforms at 145th Street and South Ferry are too short to accommodate full-length ten car trains; as a result, only the first five cars of the train open their doors at those two stations. Similar selective door operation protocols are used on many commuter rail lines within the Northeast megalopolis since some commuter rail stations have platforms that are too short to accommodate longer trains.

Many modern Selective Door Opening (SDO) systems are operated / controlled by Global Positioning System (GPS), which locates the train at the specific station. As the train pulls into to the station the GPS identifies the train's positioning and tells the SDO control (which is located on the train) what station it is currently at. This then enables the correct side of the train and enables the doors on the correct number of coaches to be opened, thus aligning the length of the platform.

...
Wikipedia