DistantRumble,
With ref to the Heathrow Shuttle and the fact that it always leaves on time. That's because:
1) There is no airbridge to withdraw and secure.
2) No hermetically sealed door to be closed in the correct way or else the train cannot move an inch.
3) No reporting of these events to other people via radio or telephone.
4) No tug or chocks.
5) No ground walker to assist during push-back.
6) No engines to start.
7) No taxi.
etcetera.
When the time for departure arrives, the train operator (not a driver as it is automated) looks along the platform and pushes the button to close the doors. If someone is caught in the doors, they half open them and close again.
They then push a button and the train departs on a dedicated track where the previous departure was 15 minutes earlier. OF COURSE it leaves on time!
OF COURSE it's frustrating to get on board and get a delay but that delay has not been generated by the flight and cabin crew.