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Due to open in 2020, the tunnels of the Northern line extension will be a little bit special. They'll be the widest tunnels on the tube network.
How wide are tube tunnels currently?
At the moment, standard tube tunnels are 3.6m wide. If you've ever watched a tube train disappear into a tunnel, you'll know it's pretty tight in there, with not much space between the train and the tunnel wall — which is why tube trains can't be made any bigger.
How wide are the Northern line extension tunnels?
The 6.4km of new tunnels created for the Northern line extension will have a 5.2 metre diameter, 1.6m larger than current tube tunnels.
Why are the Northern line extension tunnels so wide?
The reason for this extra width is a simple one. The new tunnels will have something no tube tunnel has had before; a walkway.
A 1m wide path is being fitted alongside the tracks to allow passengers to evacuate on foot in case of an emergency, or to allow emergency services to access the train from the nearby stations.
In September 2017, Geoff Marshall went behind the scenes in the new tunnels and found out about these walkways:
Does this mean the Northern line extension will have wider trains?
No. The Northern line extension is still part of the Northern line, continuing its route beyond Kennington on a new branch. This means the same tube trains need to be able to run on all sections of the route, and as larger trains won't fit on the older sections of tunnel, we won't be seeing more spacious trains any time soon. Or probably ever, given the logistical nightmare of widening tube tunnels across the existing network.
How were the Northern line extension tunnels built?
The tunnels are the work of two tunnel boring machines, named Amy and Helen after astronaut Helen Sharman and aviation pioneer Amy Johnson. Tunnelling took place between April and November 2017, on two 3.2km tunnels between Battersea and Kennington via Nine Elms.
Here's the moment the tunnel boring was completed:
Find out more about the Northern line extension from TfL.
*Pedant's note: we're aware that some people still maintain a distinction between 'tube trains' and 'underground/subsurface trains' but we do not. Sorry.