Picked up a leaflet on the extension of Overground to 5 carriage trains. It says that Canada Water is one of the stations with platforms that cannot be extended (Wapping / Rotherhithe also). At these stations the doors of the 5th carriage will be kept closed and an announcement made for passengers to make their way to the 4th carriage if they want to disemabark.

Boy, that sounds like a recipe for chaos at Canada Water especially in the morning peak (throw in the inadequate single escalator as well).