Complex sentence: 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.
Split into simpler sentences: 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.