It is true, it's used for access between carriages, although it's a no-no to do that on the tube unless you're staff. On the overground units though the corridors are linked up and you can walk through just fine.
I can go back a bit further though and show you this:
The back end of an A4 Pacific. Now, you can see a little door in that tender, and that was put there specifically so that on a long express train to the North from Kings Cross to Edinburgh, the crew could change half way without having to stop the train. The Scots crew would just pick up the train from wherever and sit in the carriage behind the engine, and then at the allotted place they would just go through the door, through the tender and take over from the London driver and fireman. As seen in the classic BTF production - The Elizabethan Express: