A 3D printer is a machine that makes real live things in three dimensions (duh). They’ve been around for several years, at prices beyond the wallets of most mortals. Prices have been falling as more companies get into the business. A couple of years ago, I bought one as a kit, and have it almost completed.
If you did as I did, you started collecting Large Scale equipment based on whether they appealed to you or not. A familiar railroad, a favorite industry (beer cars come to mind), an association with a time or place. One thing I did not consider was whether any two of them would actually couple together reliably.