"Work on the Tracks" is a project by Jan Sledz and Henning Tilp in Berlin.
One rail of the two tracks running through an underground station was polished until shined brightly, standing apart from the other soot covered rail. Nuts of the polished rail were covered in gold caps. All of it to call an imagination of cleanliness.
The project was the winner of the annual competition by the Neuen Gesellschaft für bildende Kunst (NGBK) Berlin, the topic of that year was "Cleanliness, Service, Security".
I really like this because instead of making anything more (sculpture, poster, sticker, or anything else that actually just litters the public space) to promote cleanliness, service, and security -- they simply cleaned the track and offer an image of what cleanliness could be. It is ephemeral, relational, and strikes the communication objective at the bull's eye.