Trainspotters are a special breed of sad people and even I don't understand them. Their entire hobby is merely to collect as many stock numbers as possible. Thats it. I cannot understand why and they are everywhere.
It is NOTHING like being interested in PC's, cars, or even being interested in railways.
However what bothers me most about them is that most of the general public think there are only two types of people - people who are not interested in railways, and trainspotters. Therefore if you happen to casually mention you've got an interest in rail transport, everyone thinks you are a member of the pointless Thermos Flask and Bobble hat brigade