A man on a night train from Amsterdam to Paris finds a sleeping stranger's head resting on his shoulder, and he doesn't want to move. That's the whole situation, and the poem lets a small, accidental contact grow into something enormous.
The first stanzas set a hypnotic scene: the train's roar fades into a "dark and silent breathing," so the carriage feels like a shared, sleeping body. Her hair lies "like a wreath" on him, a tender, almost ceremonial image. The speaker knows he should shift away and rest before his meeting with a customer, but he stays put. The list at the end shows his mind spiralling: "guilty innocence," "delectable deniability," "once in a lifetime." Each line piles on a new reason to stay, until he simply names the thing itself. The mood is hushed, secret and bittersweet, a moment he wants to stretch out while knowing it will end.