The speaker is caught totally unawares by the passage of a noisy fire truck. He has kind of a Zen moment right in the middle of the street. We imagine him walking down the street with a thermos of tea in his hand, tired and wet. Amid the streetlights and rain, he hears a gong and sirens, and suddenly he's in another reality. Maybe you've heard the Beatles song "Revolution 9," which consists of someone repeating over and over again, "Number 9 … Number 9 … Number 9." Well, the speaker has a different number echoing through his mind, "Figure 5 … Figure 5 … Figure 5."
What is it about this golden number that the speaker finds so compelling? We have no idea. Maybe he was just in the mood to be put into a hypnotic trance. He's the kind of person who can open himself up to the mystical side of modern life. In order to devote his full attention to this experience, the speaker stays out of the way for most of the poem. He only injects himself into it with the phrase "I saw," which puts him in a passive position in relation to all the sound, lights, and movement. His very sense of time seems to slow down toward the middle of the poem. At the end he returns to his senses, as he zooms back out to focus on the city as a whole.