wrote

Two buttons, in both cases.

and for that matter the DLR.

It depends what mode the train is in:

If the captain is operating the doors from the front, then it is one
button that he pushes.

If he is moving about the train however, he will close all other doors
from where he is standing. But his door will remain open and allow him to
check the platform. After he closes his door, then the train takes off --
no other positive action is required.

AIUI when operating from a door he presses one button which initiates the
closing of all the other doors. When he has checked that no-one's trapped he
initiates the closing of his door. When that is closed the train starts.
When operating from the front he presses one button to initiate the closing
of all the doors one side (and another button if the doors are open both
sides, as at Canary Wharf or Tower Gateway. When the doors are all closed
and he's checked in the platform mirror(s) that it's safe to go he has to
press another button to start the train.

Peter