Did they always have train captains ? I know there have been incidents (like a gang of kids chasing someone onto the tracks to be run over), that might have made them introduce "train captains".
I don't know how they'd do it on the tube. Presumably door closing would still require manual control.
What for? The doors can be on a timer (station-specific if necessary)
The door closing works well for the DLR -- the PSA checks all the doors are clear and then shuts them all at once. This probably isn't viable for a tube because platforms are smaller and more congested.