Because these lines share a lot of track and infrastructure, they are being modernised under a single combined and integrated project, Four Lines Modernisation (4LM).
The four lines are among some of the oldest sections of the Underground network, with parts dating back to 1863. Together they make up 40% of the Tube network, with around a million passenger journeys each day.
The trains and other equipment used on these lines are old and need to be replaced. We're introducing new technology that will reduce journey times and increase capacity - essential to support London's growing population.
Modern, air-conditioned S-stock trains now in operation across the Circle, Hammersmith & City and Metropolitan lines are gradually being introduced on to the District line. This roll-out should be completed by the end of 2016.
These trains are longer and more spacious than the old trains they replaced. The train doors are also bigger, creating extra space for more people to get on and off at stations. This speeds up journeys.
The new trains are walk-through with air conditioning, as well as improved audio and visual information, dedicated wheelchair spaces and colour contrasting interiors.
By replacing old and worn out track and points your journeys will be quieter and smoother. Coupled with brand new drainage systems, they will be more reliable during bad weather.
Better drainage means that the track is much less likely to flood, reducing delays to journeys because of signal failures, as well as fewer closures for maintenance and repair works.
Work to install a new signalling and control system began in summer 2016. This will eventually allow the trains to be driven automatically, with a train operator in the cab to open and close the doors. The train operator will be responsible for managing customer information and safety.
Similar technology introduced in recent years on the Jubilee and Northern lines improved performance. The new signalling system allows trains to be run closer together, meaning a more frequent service and shorter waiting times, allowing more people to be carried. This new technology will enable us to reduce delays and improve reliability.
The programme will allow us to operate 32 trains per hour, a 33% increase in peak-hour capacity. Installation work will require some line closures.
More frequent services will be introduced: