It was the final episode of BBC’s “The Tube” this week.

Going behind-the-scenes on the London Underground, the series has revealed a fascinating picture of life on one of the world’s busiest underground rail networks.

Episode six focused on those who start work when all others are tucked up in bed.

The programme showed the work of the various teams of cleaners, responsible not only for the stations themselves but also for scraping dust and dirt from the tracks, and vacuum-cleaning the tunnel walls. Repair workers were also featured, and the replacement of a piece of track was documented.

All of these people had to work to tight four-hour deadlines, completing their underground tasks before power could be restored to the track in time for the start of the day’s service.

As is the case in many professions, it’s the “invisible” workers such as these who may not be seen publicly but make an essential contribution by sorting out any issues which could lead to problems – and potential disruption for passengers.

There’s still time to check out The Tube on the BBC iPlayer if you’ve missed it.