What happens in the New York tunnels in Mark's dream/flashback in Chapter 35?

Mark dreams memories once more. The water is flooding the subway station, and Mark, Alec, Trina, and the others sprint away. The tunnel begins filling with hot water, and Alex points out a set of narrow steps with iron railings. They head for it. They struggle to get Baxter out of the current, working together to form a human chain.