We aim to do as much work as possible during the four hours a night when the Tube is closed. But we may need longer for large projects when we must move in lots of heavy equipment.
The weekend - when there are fewer customers travelling - is often still the best time for us to work. Weekend closures give us more than 50 uninterrupted hours.
New ways of working, including the latest engineering techniques, help as we try to keep disruption to a minimum. We will always ensure there are alternative ways for you to make your journeys.
David Waboso, London Underground's Capital Programmes Director, explains:
'We know it can be disruptive but the weekend is the best time for us to work as we have more than 50 uninterrupted hours to access the tracks. Compared to weekday use, there are fewer passengers - even though we know it doesn't always feel that way.
'The four hours or so we have available at night during the week, in the time between the last trains returning to the depots (at around 01:30) and first trains starting (at around 05:30), just isn't long enough to carry out projects such as replacing track or installing new signalling equipment.
'During a weekend closure we always make sure suitable alternative travel options are available, such as alternative Tube lines, DLR, London Overground, National Rail and replacement buses.'
We work closely with other transport operators when considering closures. This includes Network Rail and train companies running services into London.
We take into account:
We always try our best to avoid closing several lines in the same area. However, sections of some lines share tracks or signals, so work on one line may force the closure of another.
Our network is a lot bigger and more complex than most other metro systems. For example, we have 249 miles of track compared to 133 miles in Paris or 56 miles in Singapore. Many metro systems are much newer, and some (like New York) have four-track systems allowing trains to run 24 hours a day at the same time as they do improvement work.