EDIT:
In your modeling program, the coupler striking face should be set at world coordinates x=0, y=0, z=0, as in this photo (the little white ball is 000). In the photo you can see that the face is on the ground. You will set the height of the coupler in the vehicle Blueprint.
Coupler1.JPG
The height I use is .8763m, which equals out to about 2'10".
As far as making your own, first you got to know how to use a modeling program, such as Blender, to make the shapes. And you need photoshop experience for the textures.
For references, there are actual coupler schematics that you can get by googling. Type e, type f, just about any you want.
When you make them, you will need two of them. The first, an open one, with the coupler open, the brake hose slack and hanging, and the pipe valve handle in the closed position. The second coupler will be the closed version with the hose in it's connected position and the brake valve handle open. When you have those two made, then you will insert the references to them into a coupler blueprint, that will be referenced by your rail vehicle main BP.
For the connecting hoses, the most important part of that is making sure that connector is centered at world coordinates X-0, Y-0, so that the couplings will meet when you have two cars mated up. As far as height, for independent developers, I believe the BritKits hose heights are the standard for connections. Those values are the ones the SMM/VRC couplers use.
Coupler2.JPG
It should also be noted that the coordinate values shown are for 3DCrafter, where X is sided to side, Y is up and down, and Z is front to back. In Blender and Max (I think), X is side to side, Y is front to back, and Z is up and down.
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