A trainspotter, someone who braves rainy and windy station platforms to catch a glimpse of unusual trains. An unproved etymology holds that this word comes from a humorous pronunciation of “grouse”, making the connection between the supposed resemblance of trainspotting to grouse-shooting. The verb grice and the participle gricing are back-formations from gricer. This is from the website for Times Online and is my sense for the word as it is currently used. I would drop the word 'unusual' as a more generic definition would focus on fascination with trains as evidenced by gricing, i.e. wandering about the planet to see, ride, and photograph them.
Several gricers were oohing and aahing with big grins as the heavy Chinese steam locomotive roared by in high dudgeon with a long line of ore cars in tow.