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1.
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.
 
2.
Person who enthusiastically collects numbers of traction engines or railway locomotives.
The gricer went enthusiastically round the rally ground collecting every number he could.
by NZ Railfan March 17, 2003
 
3.
I confirm the word was in common circulation amongst Bluebell Railway enthusiasts in the employ of Southdown Motor Services(a major Sussex/Hampshire bus company)at its Brighton Head Office in the late 1970s. These persons subsequently organised a transport "event" (held at Hove Town Hall)named "Gricerama" - so the origin propounded above may well be correct
The bus station was impeded by the usual crowd of camera-hung gricers
by D C White September 08, 2006
 
4.
The word was first coined in about 1970 at the Bluebell Railway when a group of hard-core railway enthusiasts arrived one cold December afternoon to look around. In a removals van with the company name G.Ricer on the side.
It has since moved on in certain circles to apply to fans of other forms of transport eg Busses
gricers love anything to do with railways
by Ditch Boultbee December 23, 2004