The poem catches a single moment: someone looking out of a train or bus window and seeing a girl curled up on a river bank, opposite the site of Bedford Gaol. The speaker doesn't describe her in full sentences. Instead, the words seem to arrive the way things do when you pass them quickly, in fragments, and then start to play with themselves.
"Girl" turns into "grail," which suggests the speaker sees something almost precious or longed-for. "Pure tan" slides into "puritan," and "bed ford" splits from "Bedford," so the place name becomes a bed and a river crossing. The sounds keep shifting until they reach trucks, the initials BP, and finally the nonsense word "Peeb."
The mood is quiet and a little dreamlike. The image of the girl, soft and fern-like, is gradually crowded out by the ordinary, noisy world of traffic and signs. Judging by this drift, the poem seems to be about how a moment of beauty is glimpsed, then lost.