In pictures: Inside the Northern Line
The team gathers on the Northern Line platform at Euston Station for a safety briefing before they head down the tunnels to begin their work.
The Protection Master needs to make sure that the electrical current on the tracks have been switched off. The tests are always witnessed and need to be signed off.
The cart is used to transport materials and tools to and from the site of the job. When sleepers are replaced a lot of earth and debris needs to be removed from the tunnels.
The view a Tube driver gets when driving along the tracks. It gets hotter at night because there are no trains to move the air around and as a result it just 'bakes' in the tunnels.
Project Manager Gavin Silvey in the storage room, a small area just off one of the tunnels, where tools and equipment are kept.
The team arrives at the site of the night's job where several sleepers will have to be removed or repaired and replacements put in.
Heavy drilling work is required to dislodge and remove the earth around the sleepers so that they can be replaced. Sleepers may have been in place for up to 20 years.
All the team are wearing Personal Protective Equipment (PPE) consisting of hard hat, ear defenders, safety glasses, high-visibility vest, midsole boots and, if necessary, gloves.
Members of the team take turns using the drill to avoid the risk of sustaining 'Vibration White Finger' which can lead to permanent numbness of the digits.
As well as replacing sleepers, typical maintenance work can include tightening of fastenings, bolts or clips, repairing joints and weld-repairs to crossings.
Several heavy bags of earth and waste will need to be removed from the tunnel and taken back up to ground level when the work has been completed.
There is a only a small window of opportunity every night when the trains are not running that the work can take place. Within a couple of hours trains will be running on these tracks again.