The iconic blue bar and red circle logo of the , also known as the roundel, is a symbol recognised not just across the UK but globally. It's the beacon people search for when navigating their way around the capital. This emblematic logo graces popular souvenirs such as mugs, t-shirts and keyrings, purchased by both locals and tourists, and has even influenced the design of logos for other subway systems worldwide.

However, the evolution of the roundel into its current form has been a gradual process, and in one central London Tube station, the modern-day design with the red circle is yet to fully take hold.

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Regular users of Moorgate station, served by the Northern, Metropolitan, Hammersmith and City and Circle lines, may have noticed that the station hasn't fully embraced the famous logo typically associated with the Tube.

Instead, the station sports a unique, diamond-shaped roundel behind the blue bar, rather than the customary red circle.

While this might seem unusual, this sign actually predates the circular version, reports . The diamond-shaped sign harks back to the era of the Metropolitan Railway, the precursor to the London Underground, which operated London's underground rail network from 1863 to 1933. In fact, it was the world's first subway system.

The Metropolitan Railway fittingly used a diamond-shaped roundel in its logo to artistically portray its significance to global public transport.

However, when the Metropolitan Railway joined the London Passenger Transport Board and was renamed the Metropolitan line, the diamond logos started being replaced by round ones designed by Edward Johnston in the 1920s.

But in 2013, the diamond-shaped logos were put back up at Moorgate station to mark the 150th anniversary of the founding of the Metropolitan Railway.

The diamond-shaped roundels were left at Moorgate to remind commuters of the history of the Tube, and to this day, Moorgate remains the only Underground station in London to have them.