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Selective Door Operation

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Subject: Maryland railway station, Royal Albert DLR station, British Rail Class 377, Elverson Road DLR station, Gallions Reach DLR station, Tisbury railway station, SDO, British Rail Class 395, Hayakawa Station
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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.

Purpose and operation of SDO

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.

UK variations

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 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-selected all carriages behind the location in the carriage 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 were fitted with SDO. This was unusual in the fact it required the Driver to run the leading coach off the platform. The Driver then operated the SDO system before releasing the train doors. The result was the leading carriage only was de-selected. The Guard then closed the doors from an appropriate location. These units are currently in use with Cross Country Trains, without any need for this form of SDO.
  • Class 350, 444 and 450 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 or 450 set for example, made up of two units as each unit has 4 coaches, can have one set de-selected. The Class 444 is made up of 5 coaches per unit but the principle is the same.
  • First Great Western High Speed Train 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 panel throughout the train. The Guard can then either de-select doors in front of that location or behind.
  • Most Electrostars (Bombardier built) Class 375, 377, 378 and 379 have SDO systems fitted and operate in the South eastern region of the country. All these vehicles' SDOs are controlled / operated by Global positioning satellite (GPS) systems. The Class 377s in use on Southern and Thameslink networks have an additional Tracklink II system to augment the GPS system. The Tracklink II system consists of a balise fitted at short platform stations which sends specific data to the passing Class 377 showing exactly which station it is approaching and the length of the platform that it is entering. The Tracklink II system is required due to GPS not being accurate enough to determine exactly which platform the train is entering at a multi-platform station.

Control by GPS

Many modern Selective Door Opening (SDO) systems are operated / controlled by Global positioning satellites (GPS), which locate the train at the specific station. As the train pulls into to the station the GPS identifies the trains 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.

Local door operation

Main article: Local door operation

Selective Door Operation should not be confused with Local Door Operation (LDO), used on many trains for the benefit of train crew and other staff. It could be argued that SDO and LDO are essentially the same system, but in terms of the railway Rule Book they are treated separately.


  • Hima-Sella Ltd Tracklink II system
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