"Yes, I knew what they were up to" is a strange way to begin. The "Yes" sounds like an answer to a question we never hear, as if the speaker were being interrogated, or were arguing with his own conscience. We never learn who "they" are or what they planned. The line "Fewer people... not a bad idea" hints at something violent that would thin out a crowd, but Pratt gives us only the speaker's attitude toward it, and that is what the poem is about: how a person who has switched off his sympathy is switched back on.
The first stanza is built from contempt. The syntax is clipped and half-finished: "my life barren enough / in the gray of this city." The missing verb makes the thought sound like a grievance he has recited many times. "Gray" drains the city of colour, and "barren" suggests something sterile and unproductive, which matters when a child arrives in the next stanza. The grievances pile up like a commuter's list: "the damned trolley, / every day the stinking crowd." The crowd is reduced to a smell, with no faces in it. Then comes the coldest moment. "Fewer people–maybe not / a bad idea, in the long run." The double negative ("not a bad idea") lets him avoid saying what he means, and "in the long run" makes mass harm sound like a sensible policy. The dash catches him mid-hesitation, as though he is talking himself into the idea.
Then comes "But," and the poem turns on a sound rather than an argument. The voice is a "baby voice," and it is heard before it is understood: "I don't know what she said / but I knew she was scared." This echoes the opening line, where he "knew" what the plotters intended and did nothing. The poem sets two kinds of knowledge side by side. One is information, which he can coolly ignore. The other is the recognition of fear in a voice, which he cannot. The simile "like my daughter's / just learning to speak" matters because it turns one anonymous child out of the "stinking crowd" into someone he can feel responsible for. The child is not his own, but her voice is close enough to his daughter's to reach him.
The ending is the poem's boldest choice. "I didn't think–just did / what any decent father would do." We never find out what he did. Pratt leaves "did" without an object, so the reader has to imagine the act, whether it was a warning, a shout, or a body thrown in the way. Judging by the poem's silence, the act matters less than the reflex behind it. The dash works as the leap itself, since there is no time for thought between "think" and "just did." The phrase "any decent father" is double-edged. It claims decency as natural and universal, yet it comes from a man who a few lines earlier was shrugging at the idea of a smaller crowd. It can be read as modest or as defensive, and perhaps as quiet self-reproach, since he did not feel that decency until it was forced out of him.
The form is as plain as the voice. There is no rhyme and no regular metre, only short lines that sound like speech. The stanzas run to eight lines and then seven, so the poem stops a little sooner than we expect, which suits a speaker who has no time left. The small word "but" frames the whole poem. It ends the first line ("up to but"), where it leads into indifference, and it begins the second stanza ("But it was a baby voice"), where it leads into action. The colon after "I didn't much care:" promises an explanation, and what follows is a list of petty complaints that feels flimsy once the child speaks.
The poem is significant because it does not pretend the speaker has become a better man. He still calls the crowd "stinking," and he does not announce a change of heart. What moves him is one scared voice, not a principle. That makes the poem more honest than a tidy conversion story, and more unsettling. It suggests that our decency may lie in what we do before we have time to think, and the reader is left to wonder what they would have done on that trolley.