A quick thought (oh dear!)

To add to the "because I was bored" label, I was looking at the map of the underground in the back of my diary and for some reason wondered if it was possible to traverse every tube line by changing at every station (i.e. not staying on any of the trains for more than one stop).  I found one way if you count the DLR (it goes in a tunnel into Bank so I think thats okay) but couldn't do it without it.  Theres the difficulty of teh Waterloo & city only having two stops and the East London Line being very short.  Anyway, don't read this, there should be more interesting stuff on this page.

You could also get on at Old Street to add the other arm of the Northern Line to the trail.  If I do this I'll update this.