I guess I'm not convinced that the analogy between 'fine art' and old trains is a very good one - except in the case where you might have something close to a one-off. Which, of course, raises the point of how does one know whether that is the case. Its certainly clear that almost all of the trains most of us own - while having sentimental value - aren't worth very much as collectables. As someone else suggested, it might be worthwhile to post pictures and see if you have that one-in-a-million rusty toy train that you'd 'damage' by trying to fix it up.
I think a better analogy is old cars (say a '67 Mustang or '57 Chevy Impala two door coupe - or better, a '53 Corvette). So you find one of those in someone's old barn - do you restore it or keep it around as a rusty, non-functioning hulk for the sake of posterity... (self-answering question, it seems to me). I'm reminded of watching Antiques Roadshow when the old furniture would come on and the first concern of the appraiser was whether the furniture had been 'restored' (oh, the angst and pearl clutching - and condescension in the tone of the appraiser when it appeared that was the case). As a woodworker, much of the beauty of a piece is the wood itself - along with, of course, the style and craftsmanship. If that wood beauty is buried beneath a dark, oxidized finish (that the original woodworker would have long ago stripped and restored), then - for me - its a pretty easy call. Of course, that doesn't mean firing up the belt sander with a 60 grit belt as the first step...!
