I must admit I didn't buy my 3d printer for doing model trains, but now I use it for that almost exclusively. Today I'm doing windows. I got some old models out, houses in need of refurbishing, and decided to redo all the windows, because, well, I'm staging my layout in summer, and none of the windows are open. In the early fifties, how many houses (especially in the rural coal mines) had air conditioning? I want my windows open, at least some of them. So I printed up windows that can be modeled in various open positions.
The white painted ones are for company houses. The triple is one I will have to duplicate many times for a coal breaker, and where have you seen a commercial one like that? (And yes, I can open those, too, bottom and top.) Those are 3d printed, using a Prusa I3 MK2S, which is a fused filament fabrication (FFF) printer. Now, I will tell you, printing detail is tough. I'm using a 0.15 mm nozzle, whereas the standard size is 0.40 mm, and a layer height of 0.05 mm. It takes a bunch of practice and a lot of iterations before you get it right. However, I think the end product is worth it. This stuff is not from the internet; I do my own designs, using AutoCAD. I'm not sure if an internet program like Tinkercad (also from Autodesk, the makers of AutoCAD) can do stuff like this, but an experienced Tinkercad user may be able to tell you. And it was hard finding a filament I liked for small detail work. Most printers today use a 1.75 mm filament, which you can get from a number of sources. I prefer Paramount. That's my experience with FFF printers. Not sure if one of the 'cheap' ones will do the really small stuff; check the specs. (My Prusa, at about $700 for the kit, was a lot cheaper than the $2500 MakerBots I purchased for my school, but will run rings around the MakerBots! Doesn't have the bells and whistles, though. And they won't print fine details.) And I like PLA filament for the detail work. It's more brittle than ABS and PET, but it is so much better to reproduce detail, even down to 48 NBW's and door dogs on my P-5 smokebox front. And that's all in HO scale, too.
For me it was well worth it, as I've printed a whole locomotive so far and working on two others. Now I'm doing structures.
New frame/Hodges trailing truck for Reading M-1.