Confused? You will be...
The Northern Line is a rather strange amalgamation. It's much more complicated than other lines. There are basically five sections:
- Morden to Kennington
- Kennington to Camden Town via Charing Cross Station
- Kennington to Camden Town via Bank Station
- Camden Town to High Barnet (branch to Mill Hill East)
- Camden Town to Edgware
Trains can go along any two of the sections, so listen for announcements of the form 'This train is for Edgware via Charing Cross' to make sure that you're not going to get lost.
The current rolling stock is leased rather than bought; an early example of PPP on the Underground.
The Northern Line has a long and complicated history, which is fully explained on CULG: http://www.davros.org/rail/culg/northern.html and there are some interesting facts on thetube.com at http://www.thetube.com/content/faq/lines/Northern.asp
Mornington Crescent is on the Northern Line.
Modern mythology says that the last carriage in a Northern line train is (particularly at night) a mobile singles bar: where to go if you're looking to pick someone up.