Edward Thomas begins by half taking something back. "Unless it was that day I never knew / Ambition": the sentence is hedged before it has properly started, and the doubt hangs over everything that follows. What he describes is one frosty March morning, and the odd claim is that this was the only time he felt ambition at all. It has nothing to do with careers or goals. It is a feeling of godlike mastery over a scene, enjoyed while doing nothing whatever.
The morning is built from sound before it is built from sight. Jackdaws "shout and float and soar", and one, racing alone, becomes "a black warrior" hurling "challenges and menaces to the wide sky". Ambition first appears as a bird's defiance, in martial language. Then a woodpecker's "loud long laughter" ridicules "the sadness of the owl's last cry", and the stretched vowels of that phrase let us hear the laugh. Night's mournfulness is mocked out of the day. Human life is almost absent: "all the folk astir" register only as "plumes of pearly smoke", while the frosted meadows are "happier / Than was Elysium". That is a grand classical comparison, dropped casually into a valley scene.
Then comes the train, which "roared along" and drags behind it "a motionless white bower / Of purest cloud". The paradox at the heart of the poem is that the machine is loud and moving, while what it carries, apparently its own steam, is still and silent. It is "So fair it touched the roar with silence." The speaker's response grows steadily larger. He could sit and "think I had made the loveliness of prime, / Breathed its life into it and were its lord". The verbs are those of creation and possession, and he soon declares himself "Omnipotent". The sharpest touch follows: he does not even deplore "That I did nothing." This ambition is satisfied without effort. The power he feels exists only because he has stopped wanting to act.
The form quietly supports this. The poem is a single unbroken block of twenty-six lines, mostly iambic pentameter, with rhymes that chain forward like terza rima without ever settling into tercets: "knew" and "blew", "high" and "sky", "tower", "hour" and "bower". An -er or -our sound recurs again and again ("before", "soar", "warrior", "woodpecker", "astir", "happier"), so that one note seems to keep ringing. Sentences also spill over line endings, and the line that says "Time" stops on the word itself, so that time appears to hang there before the next line begins. The moment of stillness is rendered in the syntax.
Then the spell breaks. "But the end fell like a bell" is a twelve-syllable line, longer than any around it, and it tolls. The bell sound carries into the final rhymes, "tell" and "well". The scattered bower and the distant train, which "roared" again, bring back the noise that silence had briefly overcome. The last two lines nearly repeat each other: "But if this was ambition I cannot tell. / What 'twas ambition for I know not well." The second line does not answer the first, and it shrugs. The opening "Unless" has become a closing "if", and the poem ends where it began, in uncertainty.
That refusal to tidy up is what makes the poem worth reading. It takes a word usually tied to striving and suggests it may be closer to a brief, wordless sense of possessing the world, one that vanishes as soon as it is noticed. It also avoids easy moralising about the railway. The train is noisy, yet it creates the loveliest thing in the poem, and it destroys it simply by moving on. By admitting that he cannot say what his ambition was for, the speaker makes the feeling more convincing. A confident definition would have explained the moment away, and the poem leaves it intact.