Well, with a collector like you, something would be considered obscure if it's not in your collection!
But In all seriousness, I would say makers that aren't normally targetted for large collections. Such as makers of low quality, or ones that only produced for a short amount of years, or ones that are hard to find information on, or are completely off the radar. Maybe a prototype of some type, or an exclusive train for a special event. Custom-made for a high paying customer, or a gift to a valued employee. Anything along those lines.
-Berk
