Why is the opening of 'The Idiot' so powerful?

"Towards the end of November, during a thaw, at nine o’clock one morning, a train on the Warsaw and Petersburg railway was approaching the latter city at full speed. The morning was so damp and misty that it was only with great difficulty that the day succeeded in breaking; and it was impossible to distinguish anything more than a few yards away from the carriage windows."

And then we are inside the train . . .

It is so powerful because at the very beginning, words, which are after all disembodied spectres of the intellect, do not come to us as such, but as concrete sensuous images. The reader's eyes are not focussed on the words on the page, or are only mechanically so. The reader's eyes are a movie camera. The page is the screen upon which we watch the scene, and we hear the attendant sounds. And within ourselves, we also smell and feel some of the things.

The beginning of 'Anna Karenina' might have been equally perfect with its marvellously simple in medias res sentence: "Everything was in confusion in the Oblonskys’ house." Oh, but dear old Tolstoy! It is believed that he later added those celebrated, flabby lines which now open the novel—"Happy families are all alike; every unhappy family is unhappy in its own way." And he spoiled it, as Thomas Mann once gently observed.