manned but unmanned

Engineers usually incorporate redundancy or factor of safety into their designs. It can be as simple as knocking a few more nails into the plank, or as sophisticated as having a few backups. But where to draw the line between too much and too little redundancy?

The Alstom trains running on our North East Line and Circle Line use something called communication-based train control. Or to put it simply, they don't need drivers in the train. Fully automatic. However the operating companies still station a staff to stand at the front window of the train.

I have no issue with the trains themselves, they are efficient, punctual, close doors on time, but it's the stationing of the staff that I am puzzled with. Is the staff redundant? (that is a weird question to ask, especially from someone with an engineering degree. Of course he/she is redundant!) If they have to station a staff on each train, wouldn't it be cheaper (I presume) to just go with a manual non-automatic train and the infrastructure?