Materials Resin used for model trains

You are absolutely right that not all resins are made equal. And it's not just a matter of quality.

The way I usually explain it is that a BJD is made from resin the same way a cupcake is made from batter.
The word resin in this context only really means that it's a semi-viscous liquid. There are many types of plastics that can be cast using resin. There is polyurethane that is what the BJD considered on topic for this forum is made from. Then there is polyester, acrylic, epoxy and many more. Some of these are avaialble in other forms than resin, best known within this hobby is probably epoxy that is often used in the form of putty as a scultping material for making mods.

My best advice is to forget about that word for now and start to look into what material you want your finished doll to be made of. Then go from there to figure out how you want to go about it.

 

That is going to depend a lot on what conditions you are working under and what equipment you have access to. To name an example: all polyurethane resins are sensitive to humidity, but at different levels. If the humidity is too hight there can be issues with the curing as well as with gas bubbles forimng as the resin cures. This can be compensated for by contoling the enviroment in your workshop as well as by using a preassurized container for the curing.

There are many variables to consider, so it's not really possible to give you any easy answers. My best advice if you want to do your own casting it to just start experiment. Make a simple project with just a few simple pieces in roughly the same range of volume and complexity as the pieces of the doll parts you eventually want to make and take it from there*. You'll still have both the finishing of your originals for casting and the entire moldmaking process to learn before it's time for your first cast. By then you'll probably have a much better idea of what properties you are going to need and probably have worked up some good habits for doing material tests and setting up a good workspace. Be prepared for a bit of experimentation and don't be afraid to test out several different resins from different makers to see what works best for your process.
If you get stuck, post about it descibing what went wrong and I¨'m sure people here will help you trouble shoot.

*The reason for making a simpler test project is to make it less of a disaster if it gets ruined. Not everything is going to turn out perfectly every time and when doing something for the first time it's always extra difficult to predict everything that can go wrong and plan around it.