On Mon, 1 Jan 2007 19:21:34 +0000, Tom Anderson
wrote:

While we're sort of on the subject, why do tube lines normally close at
night? Maintenance of some sort, AIUI, but what, exactly? What is it that
requires a six-hour closure of every line every night?

They need the night time closures in order to break just enough
District Line track to completely wreck Monday morning services of
course!