So, you take a suitcase. Inside, you put a Mac mini, a portable projector, and a heavy duty battery. You attach suction cups to the outside of the suitcase, and cut a hole so it can project outward.
When the subway comes, you stick it to the side of the train, and you have suddenly have miles of underground canvas and a captive audience. Here’s the video (from Berlin).
I’m not sure the security people would be too crazy about you attaching a suitcase to a subway car, but I wouldn’t be surprised to see advertising done this way.