When it comes to Monorails its all about perspective. If they are going frontwards then the back is the back and the front is the front, but if they go backwards then the back is the front and the front is the back. So if the monorail in the picture is going frontward, then that is a picture of the back, because the back is the back. If its going backward, then its a picture of the back, because the front is the back. So either way, it's the rear. (of course the same argument works for the front end, and it doesn't make any more sense that way.)
None of this really matters though, as long as they say "Por favor, mantegan se allejado de las puertas."