I don't think the problem is having an interest you're passionately involved in or necessarily talking about that subject to others, if is how you do it. My uncle is into birdwatching, not my cup of tea, bit when he talks he talks with such obvious love for the subject and how he enjoys how a certain bird flies or moves. I could listen to him talk for hours. I think as a whole as railway enthusiasts are a) not always as enthusiastic about the railway as we should be b) not always as gopd as they should be about explaining what they fkmd exciting and engaging qblit their hobby.
Mmm -- I suspect that these things are "situational" in various ways -- including "how you (can) do it". There's a very wide span between different people, as regards being word-of-mouth articulate / being entertaining talkers... or not. I suspect that if there were a real hobby which revolved around watching paint dry; someone with a sufficient gift of the gab, could talk about it for hours to people who didn't share the hobby, and have them spellbound. On the other hand, with my brother -- who is a great guy, with very many excellent qualities -- he's not very articulate, and with a conversational style and delivery which is, unfortunately, fundamentally dull. His chief interest in life, and topic of conversation, is what he does for a living -- he's an engineering draughtsman; he can and often does go on for almost-literally hours, about this, and various work-type situations which involve him. Sadly, his way of conversing is such that it all comes across very uninterestingly to us, his other brothers -- and a lot of the time, the details of it all are extremely obscure to us -- we try to listen with some interest, but that is very hard, and we find ourselves heartily wishing that he'd shut up / devising stratagems to try to change the conversational topic.
Other personal dynamics can play a role, I feel. Perceived "monomania" can irritate people, other factors aside. A late uncle of mine was a very lively and interesting talker, essentially on a multitude of subjects; but he and his wife, for about the second half of their lives, got "into" basset hounds -- owning as pets, and sundry other aspects of the basset "fancy"; it quickly became, in the rest of the family's perception, an obsession on their part -- taking over their entire lives. Rightly or wrongly, rest of family tended to have little use for his talk about basset hounds (charming creatures though those are) -- no matter how entertaining his delivery: reaction -- in thought, if not spoken -- was very apt to be, "Uncle, enough with the bassets, already !"
I'd imagine that your uncle is a fairly lively and articulate talker in his own right -- I don't know him, of course, and you do ! I just figure -- many "variables" along the boring-to-fascinating continuum.
