You're on the Green Line - quick, which car are you on?

On Commuterrant, Ellen McCoullough wishes the T could come up with a different way to tell people in the third car that their train is too long for Arlington Street station and they need to head to the front of the car to get off:

... I ask you: how am I supposed to know if I am on the third car or not? My geospatial skills are not that refined, but it did become obvious when the train pulled into the station and I saw only the darkness of the tunnel outside the mid-car exit. It was then that I used my powers of deductive reasoning to determine that, indeed, I was on the dreaded third car. ...