That is a fascinating paragragh. I had not noticed that before. What a marvelous image: two opposing sheets of darkness threaded together by the train. Actually not threaded together but being threaded through the train's motion. And her pale face contrasts nicely with the black sky, making it so vivid. I can't help but think this is a sexual allusion. Her pale face waiting for the snake to thread them together.
Sorry about that, but that's what I think Lawrence is suggesting. Actually, unless it's my dirty mind playing with me,
there's a lot of sexual suggestiveness in that paragragh. I won't spell it out because then I'll really blush.
Perhaps this is where the witch's charm is placed on him. So here's a question. Is the charm that Wini puts on Coutts a sexual spell?