Among the rain and lights (lines 1-2)
So many things happen simultaneously in the poem, but they are revealed to us one by one. For example, what is the effect of putting the detail "and lights" on a separate line from "the rain," if these two things go together?
firetruck moving tense (lines 6-8)
In the middle of the poem, you find this tension between movement and frozen time. The fire truck is "moving," but its motion is "tense," like a contracted muscle. Time condenses in the middle of the poem. The lines are shorter, which means you generally read this section more slowly.
to gong clangs (line 10)
Williams creates a sense of time passing by adding new details, like this flurry of sound. But we're not sure whether time has actually passed or not. Surely the gongs and sirens were going off the whole time?