Editing Northern line (section)

===Tunnels===
Although two other London Underground lines operate fully underground, the Northern line is unusual in that it is a deep-level tube line that serves the outer suburbs of South London yet there is only one station above ground (Morden tube station) while the rest of this part of the line is deep below ground. The short section to Morden depot is also above ground. This is partly because its southern extension into the outer suburbs was not done by taking over an existing surface line as was generally the case with routes such as the [[Central line (London Underground)|Central]], [[Jubilee line|Jubilee]] and [[Piccadilly line]]s. Apart from the core central underground tunnels, part of the section between [[Hendon Central tube station|Hendon]] and [[Colindale tube station|Colindale]] is also underground. As bicycles are not allowed in tunnel sections (even if no station is in that section) as they would hinder evacuation, they are limited to High Barnet – East Finchley, the Mill Hill East branch, Edgware – Colindale and Hendon Central – Golders Green.<ref name="bicycle">{{cite web |url=http://www.tfl.gov.uk/assets/downloads/bicycle-tube-map.pdf |title=Bicycle on Tube map |date=June 2013 |publisher=Transport for London |access-date=5 April 2015 |archive-date=2 September 2013 |archive-url=https://web.archive.org/web/20130902230247/http://www.tfl.gov.uk/assets/downloads/bicycle-tube-map.pdf |url-status=live}}</ref> There are also time-based restrictions for the sections where bicycles are allowed.<ref name="bicycle" />

The tunnel from Morden to East Finchley via Bank, 17&nbsp;miles 528&nbsp;yards (27.841&nbsp;km),<ref name="linefact" /> was for a time the longest rail tunnel in the world. Other tunnels, including the [[Channel Tunnel]] that links the UK and France, are now longer.