6/6 Following the workers who descend on the Tube each night to maintain, repair and clean it.
5/6 The Tube's most unusual employee, a hawk called Toyah, is put to work on pigeon patrol.
4/6 Upgrading the tube is not easy, with both new trains and new signals causing problems.
3/6 Two drivers face their worst fear when someone falls - or jumps - in front of their train.
2/6 Following ticket inspectors as they pursue fare evaders across the tube network.
1/6 How does the tube cope with the ever-increasing numbers of weekend passengers?