If you've travelled on the latest models of London Underground trains, you may have noticed not just the walk-through carriages, modern seating and smart passenger information screens, but also buttons on the doors.

While it might seem unnecessary given that nearly all Tube trains automatically stop at every station, there's a reason for these buttons.

These door buttons can be found on the 'S' stock trains running on the Circle, District, Hammersmith and City and Metropolitan lines, as well as the 1992 Tube stock trains on the Central line.

They're also set to feature on the upcoming 'New Tube for London' trains due to hit the Piccadilly line later this decade.

Apart from the 1992 stock Central line trains, all these trains are air-conditioned, meaning the carriage air is usually co

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