I am the opposite of Tom when it comes to hand fabrication... when I've made a "master", I've made my part usually and have all I need. I last tried resin casting 40 years ago with that stuff you used to make paperweights with pictures of your grandkids inside (although at the time I was a grandkid making them for my grandparents). Needless to say the results were not usable and I never really messed with it again.
Much has changed since then and the materials are vastly superior to what was available 40 years ago. Could I learn to make resin masters and molds and do casting? Sure. Do I really need another hobby to go along with carpentry, electronics, photography, computers, painting, sculpture, chemistry to name a few that have something to do with model railroading... in addition to those that don't like vintage cars, playing music, punching holes in paper, being chased by terriers, landscaping, doing laundry, and inventing new ways to remove mud from hardwood floors? I learned a long time ago that I can do anything I want in life, but I can't do everything I want. Although I'm probably now too old to be the first fat guy in space.
A lot of the parts I've gotten made were just on a whim from someone doing resin or masters overhearing me talk about something and saying hey, that's a good idea. I come from the days of styrene transplants and I still do it. If I need Pullman windows I could make a resin master and mold, or I can just cut them out of the dozens of Branchline Pullman sides I happen to have (thanks Bill). While there may be more efficient ways of accomplishing some of my goals, I'm not necessarily in a big hurry to replace an activity that I do well with one I'd have to start from scratch to learn. It happens, but at this point in my life any time I put a new marble on the table I can expect to hear another marble or two fall off someplace, without necessarily knowing which one(s) or where.
When I was young, the only thing stopping me from building the world's largest and coolest model railroad was money. Now it's more about time and opportunity cost in terms of time. And no longer wanting to build the world's largest and coolest model railroad - just mine.