When I was a child the front side displays on new Muni buses used to use these probably solonoid driven LED arrays. If you sat under one you could here this clattering sound that sounded kinda like rain each time the display changed. This discussion is bringing back old memories of those.
The older Breda trains and I think buses also used to use backlit paper rolls for signs: https://upload.wikimedia.org/wikipedia/commons/4/4c/T_Third_... Those were significantly more readable