For years, I only saw Andy reproduce stories he read while playing with the Thomas trains. He could only use the character trains in the way that he saw them in the book or in the video. Lately, I have been seeing him use his stuffed animals to play the characters in his stories. True, he is still using stories he has read or heard, but he can imagine Nemo standing in for Arthur and Cat standing in for Buster.
It's said that children with autism lack imagination. I think that isn't true. Sure, they tend to be more literal but I think there is imagination lurking inside them. They just don't know how to let it come out in ways that we are used to and accept.