The term 'trainspotter' stems from exactly that......he who spots trains. A strange affliction. These are characteristically 30 - 40 year old men who still live with their mothers and quite possibly indulge in dubious sexual practices involving them. Gathering on station platforms in all weathers and wearing old anoraks and bobbly hats, they note the make, model and characteristics of each train that passes. Thus 'trainspotter' has come to mean any obsessed character who insists on talking about nothing but their chosen specialist subject. In my experience, the 'trainspotter' or 'pothouse' almost always manifests itself at the aftershow hospitality.
trainspotter as - "you are a trainspotting nutter" - In other words, someone who is psychological unstable.