London Transport Museum:
The roundel first appeared on Underground station platforms in 1908. The bar and circle, as it became known, comprised a solid red enamel disc and horizontal blue bar. These early roundels, framed with timber mouldings, were introduced as station nameboards. The new device gave prominence to the name of the station, and helped passengers distinguish it from surrounding commercial advertising.
Travelling around London, it is impossible to miss the iconic ’roundel’ in its various forms (there are different versions for the Underground, buses, and Overground among others). As is typical with most things in London, the history of the symbol is long and rich.