I don't know if anyone has noticed this, but I am just watching the third film, and I am on the scene were Doc is explaining his plan to Marty about pushing the DeLorean with the locomotive.
Clara can be seen in the film before Marty and Doc save her. She is spotted staring at the clock that's just been carried of the train.
The camera will point to her when Marty says "Right, Doc, but according to this map, there is no bridge."
Easter Egg perhaps?