[mseebach]

I believe that I read something about upgrading the signalling and/or "third rail" electrical systems was part of it, so they can shut the line down in stages behind the last train and reversely in front of the first, rather than having to wait for the train to terminate at the end of the line and stopping as soon as the first train leaves, often up to an hour outside central London. This gives one or two extra hours of work every night which is why they don't need the weekend.

But quoting from memory, could well be mistaken.