Trainspotters

After years and years, it finally occurred to me to find out exactly what the hell a "trainspotter" is. Well trainspotters, or railfans, are a classification of people that range from the amateur spotter to the obsessive weirdo. They are, as simply as the name suggests, people enamored with railways, and all aspects involving railways. Trainspotters are a particular classification of railfans that are particularly hung up on attempting to spot and identify every single piece of rolling stock know to exist for a particular rail company. They can be found hovering around rail line with note pads or tape recorded jotting down the serial numbers of passing rail cars. They are generally well connected with communities of other trainspotters in order to form as complete a picture as possible.

From Wikipedia, more on railfans:

Excessively enthusiastic railfans are sometimes referred to pejoratively as "foamers", especially by American railworkers, in reference to the joking notion that they foam at the mouth in their excitement over train operations. "FRN" (for "fucking rail nut")[2] is a similarly-used term in the American rail industry. The UK equivalent is "crank", which has been taken into enthusiast usage.

In the UK, railfans are sometimes called trainspotters, grizzers or gricers, according to their speciality (see below) or anoraks. In Australia, they are sometimes called "gunzels".

Just further proof that obsession knows no limits. The more you know.