Carriage 6, 8 or 10
In Japan they are blessed to have possibly the worlds greatest train network (Switzerland comes close) that allows you to travel seamlessly around the country at high speed and on time. To the second.
Another aspect I love about the Japanese trains are their precision, so much so that on each platform they are able to advise passengers where to queue to board their train depending on which carriage they are traveling in.
The above photograph lets passengers in carriages 6, 8 or 10 (depending on the size and type of train) know that this is where they should queue for their train.