Williams was inspired by Walt Whitman's use of distinctive American diction that approximated the way people really talk. "The Great Figure" reflects his experiments with plain American-speak. Aside from the fact that it is divided into verses, this sentence would not be out of place in a newspaper article today, if the writer were trying to add a little color. Also, Williams depicts the American metropolis – the "dark city" – in terms at once chaotic and spiritual. Big cities are places of endless distraction where you can nonetheless find Zen-like clarity. No wonder the Beat poets found Williams's writing so appealing.
The speaker tenses up as the fire truck goes by, but there is nothing particularly unpleasant in his description of the scene. He feels comfortable in the city.
The word "dark" is used in a figurative sense, to mean "hidden" or "unknown," since we already know that the city has lights.