This is the definition given to people who stand in fields waiting to take pictures of trains. These people are occasionally surrounded by animals. Hence the term "Goatshagger".
This is as opposed to the standard trainspotter whom stands on the end of station platforms.
The word was first used on the Festiniog Railway in North Wales. Loads of fields and sheep. The obvious connotation has already been used so consequently, the word Goatshagger was born.
Look out for the Goatshagger stood on the ladder in the fields below Rhiw Goch.