To be honest, in my lifetime I've seen closed carriages only in films and museums, which makes me suppose this expression was coined very long ago. Was it a foreign expression (French, for instance) which somebody translated into pt-PT and was brought to Brazil by the Portuguese? Is it a genuine Portuguese saying? Or is it just a Brazilian expression? Can you think of any contemporary expression in Portuguese having the same meaning?
"pelo andar da carruagem" translates as "the way things are going..."