Call it providential, but just yesterday I was discussing this very subject (a complete breakdown in the city's transit system) with a friend of mine who works with me, and we agreed that as soon as we got a hint of trouble on the trains we would immediately make our way to the ferry terminal.
Many of those people who are still on line at the ferry terminal aren't going to get home tonight -- the report I saw on the news is that the line to get on the ferry is now 45 blocks long.