The poem opens by establishing its disarmingly casual tone, pitching somewhere between a narrative and dramatic monologue. Though the whole poem is tightly—and virtuosically—controlled in terms of its form, the language is intentionally down-to-earth and even prosaic (as is Larkin's work more generally).
Like a story or diary entry, the poem starts by setting the scene. The poem is set on Whitsun Saturday in mid-1950s United Kingdom. This isn't an arbitrary detail, but a key part of the poem's setup. Because of tax and marriage laws in the U.K. at the time, the Whitsun weekend in May was an advantageous time to get married. It afforded certain tax breaks, and also coincided with a long weekend due the Whitsun bank holiday (Monday). That's why the speaker encounters so many wedding parties on his train journey from the East of England to London. Of course, the wedding parties themselves aren't introduced until the third stanza, making them a kind of chance encounter (and subtly undermining the idea of the wedding day as something special and unique).
The stanza form used throughout is Larkin's own, but is loosely based on the odes of John Keats. This aspect of the poem is analyzed in more detail in the form section of this guide, but here it's worth acknowledging the way that the second line in each stanza is considerably shorter than the rest (two metrical feet as opposed to five), which helps evoke the push-pull rhythm of a train alternating between acceleration and coming to a stop.
Initially, the train carriage is mostly empty. This helps the poem set up a contrast between the quiet isolation of the speaker and the boisterousness of the wedding—and this contrast provides the poem's vantage point, with the speaker able to comment on the weddings in a detached and observant way. And though there is no "sense / Of being in a hurry," the mention of the date and hour does foreshadow one of the poem's main themes—the relentless passing of time. The diacope (close repetition) in line 5—"all windows down, all cushions hot, all sense"—gives the reader a sense of the uniformity of the train carriages, which again anticipates something the poem develops later: the uniformity of the wedding parties.