Train passengers who are bored during their travel tend to lean their head against the window. To capitlize on this moment of boredom and promote the Sky Go mobile app, BBDO Germany with the help of Audiva, a company that specializes in bone conduction technologies (i.e. the conduction of sound to the inner ear through the bones of the skull) developed a special transmitter that could be attached to the train window for a completely new form of public transport advertising.
As seen in the below video, these special transmitters released the Sky Go message through an inaudible high-frequency vibration that got translated to sound by the brain.