Midnight and the drills roar into action

It's just before midnight and there is a queue forming outside the Spearmint Rhino club in Tottenham Court Road.

A few hundred yards away there is another queue outside Warren Street Tube station. Construction vehicles start to block the street and men hang around the station entrance. They are waiting for the trains to stop and the power to be switched off, so they can begin their nightly race against time to upgrade the crumbling Underground. A night of back-breaking work lies ahead: the gang of 11 engineers must break out old concrete, install new base plates for seven of the concrete sleepers which hold the track in place, and then pour new concrete between the tracks.

A cement mixer and 40 five-litre plastic containers filled with water have been loaded on to wagons. Because time is limited the concrete is made to set in just 20 minutes, and has to be mixed on site.

At 4.20am they have to stop - the tracks need to be handed back for the first trains. The night has gone well but the amount of time the gang were able to actually work on track was only two hours and 50 minutes.