Why does a railfan photographer do what he does -- and why does he do it over and over and over again? Some American fans stalk the main lines, searching for every Norfolk Southern heritage unit, say, similar to the way British trainspotters try to inscribe every engine number in their notebooks -- but I have almost no interest in diesels: I have photographed a heritage unit just once, when it had the job of following a steam locomotive around as a "helper". Some fans photo