Dan Martin of Blogstor Who interviewed Moffat about why when the Ponds got trapped in 30s NYC the Doc couldn’t just park in Washington and take the train to NYC.
“New York would still burn. The point being, he can’t interfere. Here’s the ‘fan answer’ - this is not what you’d ever put out on BBC One, because most people watch the show and just think, ‘well there’s a gravestone so obviously he can’t visit them again’. But the ‘fan answer’ is, in normal circumstances he might have gone back and said, ‘look we’ll just put a headstone up and we’ll just write the book’. But there is so much scar tissue, and the number of paradoxes that have already been inflicted on that nexus of timelines, that it will rip apart if you try to do one more thing. He has to leave it alone. Normally he could perform some surgery, this time too much surgery has already been performed. But imagine saying that on BBC One!”