Each platform has a tile design that plays on the station name. This one, of course, is Warren Street.
It's ever-so-slightly geeky to know these things are called roundels in the first place, but the real geeks will know where to find examples with unusual shapes and lettering (do you know, for example, where to find the only roundels to contain a comma?). The one above is from Maida Vale.
This is a still from BBC documentary The Tube, which reveals the secret call signs used to alert cleaners. The 'Inspector Sands' call alerts staff to a potential fire. More here.
Thanks to Vicki Pipe for the background image.
If you think of yourself as a tube geek, then you'll reassess your credentials after reading this masterclass in pedantry from blogger Diamond Geezer. TfL has since removed the pages.
Tube geeks should also check out the Station Master app, which tells you exactly which carriage to stand in for the quickest interchanges as well as 3-D station maps. It's by Londonist's Geoff Marshall who also produced these fact-filled guides to the tube lines.