While I am not trying to belittle your hobbies. But Model Trains isn't for everyone, and a lot of peoples hobbies are not about making toys.
Sure if you are making toys, then fine, If I was a 7 year old kid, and I had access to a 3d printer, I would be printing out all kinds of cool stuff, from guns for my action figures, to special Lego blocks.
But my hobbies and interests today do not require too much plastic parts. And the occasional replacement part isn't worth the expense.