He calls them subway maps, but of course that term suggests that the service is all underground, which few “subway” systems are. What matters is that they’re rapid transit. In this case, they’re specifically rail rapid transit, which is why Staten Island’s rail line in the lower left appears disconnected from the rest. In reality, it’s just connected by rapid transit of a different mode: the Staten Island Ferry.
(By “rapid transit” this blog always means transit services that run frequently all day in an exclusive right of way with widely spaced stations — linking centers to each other, for example, rather than providing coverage to every point on the line as local-stop services do.)