Selective door operation
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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.
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-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, 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 operates on the Gospel Oak Branch, although Driver Only is now in operation on the entire London Overground network.
- 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 (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.
- Great Western Railway 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 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 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 System (GPS). The Class 377s in use on Southern and Thameslink networks 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 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.
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.
Control by GPS
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.
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.