Railway enthusiast (7)
(Other definitions for spotter that I've seen before include "Train enthusiast" , "Observer" , "Spy plane" , "artist such as Seurat?" , "One who looks to train" .)
(Other definitions for trainspotter that I've seen before include "Boring person" , "Nerd" , "Gricer" , "anorak" , "One interested in engines" .)