I'm currently reading Barbara Vine/Ruth Rendell's "King Solomon's Carpet," a thriller set in part in the London Underground. (Hence my interest.) A lot of Underground trivia is mentioned, including this: 23/24 Leinster Gardens, a housing facade that hides an open stretch of track where excess steam and smoke can be vented. Looks like a real block of flats from the street, but it's not. A quick search on the Internet found this page on Cut and Cover Disused Stations (scroll to the end to find it), and a photo that you see above. Cool, eh?