If you're on the train/subway/metro/whatever, wouldn't it be great to know exactly where you were in relation to the stops coming up? Rather than knowing you were somewhere between point A and point B, this system would graphically represent where your particular car is on the subway route.
Putting the display on the ceiling is most logical for visibility's sake--and to prevent vandalism.
Implications - Many people have fears about getting lost, especially if they are in a city they are unfamiliar with. The featured idea about putting a map of stops on the subways and street cars is a great idea that many businesses can take inspiration from. Even if the business is a large mall or department store, people still like to know where they're headed.