“Honey, look at this massive model train network that just happened to be in our basement!”
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Market is hot(ish) now though, or was a few years ago. A friends dad died and he had trains. We helped ebay all of it. Owned a toy store or something, lots of rare stuff (like window displays). We even had a guy buy one of the rare posters, return it for questionable reasons, and then start selling counterfeits. Even so, the grand total wasn't a ton of money, more within the "worth doing" category.
So a collection of model steam trains might lose value, as fewer people have remember them in use, but the hobby can continue with high-speed electric trains etc.