Our upgrade of the Circle and Hammersmith & City lines includes the oldest section of the Tube - from Paddington to Farringdon - which is 150 years old.
To limit disruption, we've planned the upgrade to these lines in three stages: a new route, spacious new air-conditioned trains, and a new signalling system that will make journeys smoother and more reliable.
We're introducing 53 new trains on the Circle and Hammersmith & City lines. These will be seven carriages long - one carriage longer than our current trains.
In addition to being air-conditioned, they will have walk-through carriages, automated visual displays and station announcements, plus the reassurance of CCTV.
To improve accessibility, they will also have dedicated wheelchair areas, wider doors and walkways, lower passenger alarms and a smaller gap between the train and platform.
In 2011, we agreed a new contract - our biggest ever - to deliver a new, modern, centralised system on the Hammersmith & City line, together with the Circle, District and Metropolitan lines for 2018. This state-of-the-art system will replace a number of older systems, including manual signalling dating back to the 1920s at Edgware Road.
We know it's not something you see or take much notice of, but new signalling will allow us to run more trains, more often and with fewer failures. Combined with bigger trains that we can move more quickly and closer together, it'll mean more room - or capacity - on the line.
That's good news because as London keeps growing there'll be more people using the Tube. When the upgrade is complete the combined line capacity will increase by 65 per cent, which means we can carry about 17,000 more passengers an hour.
See a preview of the new trains.
King's Cross St. Pancras
In 2010, we completed a massive redevelopment of King's Cross St. Pancras that quadrupled the size of this key station.
It already sees more passengers a year than Heathrow Airport, with more than 100,000 people passing through the station at peak times every day.
King's Cross St. Pancras is now modern and spacious, has step-free access to all lines, and is less congested thanks to new ways to enter and exit the station.
Paddington station (Hammersmith & City)
More than 60,000 passengers currently use the Hammersmith & City station every day and demand has already increased dramatically as a result of the extension of the Circle line to Hammersmith.
We need to prepare for further increases in demand in advance of Crossrail services in 2018.