Alliteration is used throughout "The Whitsun Weddings." An early example is in line 13:
A slow and stopping curve southwards we kept.
The /s/ sounds here are distributed with an even regularity throughout the line, giving it a sense of momentum (in fact, alliteration in this poem often creates forward motion). This subtly mirrors the train's ongoing path from the east of England to the south.
In the same stanza, line 19 also uses alliteration effectively:
Until the next town, new and nondescript,
The repetition of the identical /n/ sounds evokes the way that, at that speed, each town that goes by lacks definition. That is, the speaker's eyes can hardly settle on what's there before the train is going past the next town. This also builds a sense of the cultural and historical atmosphere, referencing the numerous new towns that were built in England during the postwar years (the years after World War II).
The third, fourth, fifth, and sixth stanzas all use alliteration as way of creating a noisy, almost boisterous reading experience. This, of course, is no coincidence—it reflects the commotion caused by the wedding parties on the station platforms. The alliteration of "notice" and "noise" in line 21 shows the way that the speaker hears the guests before he consciously comprehends what's happening.
Lines 28-30 are full of /p/ sounds that, through being so obviously placed, evoke the garish and loud outfits of the girls in "parodies of fashion." The fathers wear "broad belts" (line 36) and "suits" (which alliterates with "seamy foreheads"). "Then the" in line 38 has a delicate sound that evokes the meticulous attention the girls have paid to their hairdos. "Fathers" in line 50 chimes with "farcical" in line 51 to question whether the apparent importance of the wedding days is really grounded in reality.
Once the train, now full of newlyweds, has left the guests behind, the alliteration starts to represent other elements of the poem. The /s/ sound, which was associated with the regular motion of the train in line 13, returns: "sitting side by side" (line 64); "spread out in the sun" (line 69). The two plosive /p/ sounds in line 70—"[London's] postal districts packed like squares of wheat"—are themselves packed into the line (echoed by "Past standing Pullmans" in line 73).
Finally, the poem's closing four lines intensify the alliteration to what is probably its peak. /S/ alliteration takes grip of the poem, mimicking the way the brakes are reducing the train's speed. "Slowed," "swelled," "sense," "sent," "sight," and "somewhere" brilliantly manage to evoke the sound of rain and, through their sheer repetitiveness, a kind of magic spell. This alliteration, then, neatly sums up the poem's ambiguous ending, which asks whether love is truly something magical and mythical (e.g. Cupid's arrow, discussed in the allusion section) or is as common and unremarkable as bad weather in England.