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A secret London Underground map used exclusively by Tube workers has been uncovered.

Transport for London's PAYG Map (Pay As You Go map) details all the fare information its staff need to understand the network.

Though it displays the underground and over ground layout of the city, the familiar colours of green, red and blue no longer represent the District, Central and Piccadilly lines.

The colour scheme instead represents the different charges commuters would face if they travel on these lines. The full map can be viewed here.

The green line represents typical underground Tube fares, the blue line represents other TfL fares and the red line symbolises national rail fares.

Between some stations there are white lines that represent Out of Station Interchanges (OSI), which are areas where you can walk from one station to another without having to pay an additional charge for leaving and rejoining the line.

OSI’s are time limited to ensure only those genuinely passing through on a single journey benefit. The Oyster and National rail website has detailed all of the OSI’s currently available.

A Freedom of Information request by Tim Dunning in early August resulted in TfL revealing the map.

Some of the stations on the map are also highlighted in a pink box which signifies that there are pink Oyster card readers at the station.

These readers can be used to save money on travelling fares if used when travelling around zone 1 rather than through it to your destination.

The TfL website details where each of the pink card readers are and how to ensure you pay the cheaper fare, where possible, when travelling.

The River Thames has also been coloured to signify which fare zone you are in, should you choose to use the river boat services.

When contacted, TfL explained that they currently display all fare options on their website however the PAYG Map is more beneficial for staff to ensure they have a full grasp of the network.