As far as literary texts go, this is not one that is packed with action or events. It begins as though the speaker is in conversation with another person, as he says, "Yes. I remember Adlestrop," and he goes on to explain how he knows the place—though not well. He only saw it once, from the window of his express train one day in late June.
The speaker uses imagery to describe what he saw and heard there. Using onomatopoeia as well as auditory imagery, he says that the "steam hissed" and "someone cleared his throat." There are visual images as well (descriptions of things one might see), including descriptions of the many types of trees and the "high cloudlets in the sky." The poem ends with the auditory image of a blackbird singing, though the song is described as a "mist," as though it were visual.
Close by, and round him, mistier,
Farther and farther, all the birds
Of Oxfordshire and Gloucestershire.
We can imagine what that one nearby blackbird beginning his song would sound like, and then his song becomes surrounded by a mistier, more diffuse cloud of other birds' songs, at a great distance. It's quite a beautiful and textured image, and it is delivered unexpectedly as something visual and auditory. This combination of senses in one image is called synesthesia. The impression of this setting must have been quite something since the speaker knows only the name of the town. The train didn't stop there for long—just long enough for him to form this impression. The tone of the poem, in terms of the author's feeling, is quite understated. The speaker's tone is sort of dreamy and wistful.