Underground symbol

Here, the designer uses a blue bar going horizontally through a red circle. On the map of the underground, the stations are represented by a red circle and on the each particular station's name would be written across the blue bar. The symbol is also known as the roundel, and is the graphic identity for the Underground. I think it is a visual metonym because people are familiar with the image and through way of association, can make connections with this and the Underground.