Discover the secret army battling to keep the Tube running during the twice-daily onslaught of rush hour. Travelling the equivalent of six times around the world every day, the repair workshops must work round the clock to keep trains on the tracks. At Earl's Court station, Charlotte must find a way to keep the Piccadilly Line running despite an epidemic of flat wheels on their trains.

This week, it's all change on the Underground. At Victoria, a hub station that sees 600 passengers a minute pass through at peak times, most passengers are now using contactless payment and the ticket office is being closed. Until new ticket machines arrive, a third of a million passengers at must queue up to use just four machines.

At busy Bank station, customer service assistants Eamon and Ian have perfected their own method for dealing with unexpected problems caused by passengers. They must face an epidemic of customers dropping phones, wallets and Oyster cards on to the tracks. Eamon must bring trains to a halt in order to retrieve passengers' belongings but then the train driver drops his keys on to the tracks as well.

Tube staff face their busiest, most unusual shift of the year. With a million passengers expected, New Year's Eve is the only night of the year when Tube trains run for 24 hours. Hundreds of staff have signed up for the extra shifts but they never know what the night will bring. This year, the whole network is on high alert following terrorist attacks in Paris.

The Tube is embarking on a crackdown on fare dodgers to help pay for a massive upgrade of stations and trains. Fare dodging costs the Underground £34 million a year, so revenue inspectors Simon and Paul have their work cut out for them. At Angel station, they face an aggressive passenger who refuses to pay.

Tonight, the Tube takes on more than a thousand new staff but the newbies are joining a very different Underground, one that now faces a level of terror threat unknown in the past.

With the population of London already at a record high of eight million people and expected to grow by another million people in the next six years alone, the Tube is preparing for a busier future. This time, we follow lead engineer Anita, as she digs new passenger tunnels 100ft below Bond Street station.