23/24 Leinster Gardens

I was over at Funky Pancake recently. A post there has a picture of 23, 24 Leinster Gardens in London. I didn't know about it. The secret is revealed at this interesting site. They say

The stretch of line shared by the Metropolitan and District Line between Paddington and Bayswater, much like the majority of the cut & cover lines, predates electrification and trains were originally powered by steam. The trains that served these lines were especially built with condensers to minimise the amount of steam emitted, but steam and smoke coudn't be completely eliminated. The lines therefore had to occasionally come to the surface to allow the locomotives to vent off excess steam and smoke. It is for this reason that when riding the cut and cover lines, you occasionally see flashes of daylight.
When developing this stretch of the line, it became necessary to demolish 23 and 24 Leinster Gardens, which were part of an up market street of terraced housing, forming a break in the long string of houses, so it was decided to build a false facade which matched the houses either side of the break, and use the gap behind the facade as a steam venting point.

The illusion is at first quite effective. The painted windows and doors have a similar appearance to the surrounding buildings, but on closer inspection, they contain no glass. Rather, they've merely been pained on with black and grey paint. Observing the the scene from above reveals the trickery involved!

Observing the scene from behind shows the gap between the buildings and the facade's other side is revealed to be simply a brick wall. There are many other such spaces in London. Most if not all of these are merely brick walls, behind which can clearly be heard the rumbling of passing trains. This is the most famous one, since so much effort has been put into concealing the true nature of the railway's existence just behind the facade.