Complex sentence: Class 166 units were some of the first trains in Britain to be designed for Driver Only Operation, in cases where a Guard is required they must carry out their door operation duties via a bell system to signal the Drivers to close doors and start the train.
Split into simpler sentences: Class 166 units were some of the first trains in Britain to be designed for Driver Only Operation. In cases where a Guard is required they must carry out their door operation duties via a bell system to signal the Drivers to close doors and start the train.