Selective Door Operation
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Selective Door Operation (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 electronic doors of a train separately.
[edit] Purposes of SDO
The uses of Selective Door Operation are varied.
In most country's standard practice, the conductor or guard of a train is required to acknowledge the platform staff so he or she can alert the driver that the train is ready to leave. On older rolling stock, with non-mechanical slam doors, the guard could lean out of the door's window and acknowledge the staff this way. However, with the introduction of electronic systems, train designers started using electronic door systems that had no windows. Thus guards were required to keep the doors open to acknowledge the platform staff. This caused a problem with early electronic systems, where doors were all operated together. Safety was an issue where the guard acknowledged the platform staff and then as he returned into the train to close the doors, late arriving passengers would still board which was a hazard. The newer systems have Selective Door Operation so that the guard can close all doors except for the one he is operating the system from. This allows him to step off the train to acknowledge the platform staff while not leaving other doors open to create a safety risk with late arriving passengers.
Another reason is platform length. Some trains that call at certain stations are too long for the platform. This caused an operational headache on old stock but was solved by Selective Door Operation. The guard can choose which doors are to be opened so as not to allow passengers to disembark from carriages not stood at the platform.
On buses, doors may be selectively operated when no conductor is on board and passengers are required to board at the front of the vehicle to buy a ticket from the driver. This may also be the case on buses that are operated by companies that operate a buy-from-driver policy and keep the other doors firmly closed.
