OED's earliest citation can be found here, which is from 1969.
Well, we've preserved nearly every movable object (steam, that is) on British Railways, but perhaps the greatest unpreserved loss has been the gricer, or full time railway enthusiast.
A few sources seem to indicate that gricers were fond of collecting photographs of the trains they observed, which seems to support the comparison to "grousers" who "bag grouse." It also appears to have been common for gricers to pose naked or without pants in front of the trains they photograph.
The article linked above includes this detail:
Other features of the prototype include the inevitable cameras and tape recorder: with added luxuries such as multi-coloured ball point pens, binoculars, National Health spectacles and army surplus pack.
Dawn Gill interviewed various self-described gricers in the 1990's, and also emphasized the importance of collecting photographic evidence of the trains they spot.
Gricers are perhaps best described as the punks of train-spotting. They are locamotive fans who have added an anti-establishment spin to their pastime by eschewing the mere collecting of electric passenger trains, opting instead to take photographs of the big, sexy diesel engines that thunder along inaccessible freight lines in the dead of night... The trouser-dropping is a ritual; once the gricer is in the depot and has photographed the trains, he often drops his trousers in front of the train while his friend snaps him.
Gricers do not collect numbers, they take photographs, usually of trains in depots after stealthily negotiating the obstacles of security guards and barbed wire in broad daylight. Once inside, dropping trousers or, even better, getting completely naked and climbing inside the cab and being photographed is the accepted way of proving they made it, as well as showing their disdain for those whose job it is to prevent them.
Gricers have albums full of their photographs, hundreds of trains all taken from the same angle... when one gricer agreed to send the Observer some interesting photographs for this article only one featured a semi-naked gricer. The rest were of locomotives.