Do you mean merging a catenary asset into track asset?
I thought you meant you wanted to know how to build one
it's still not that easy, I don't know if the config will let
you do that as there can only be 1 track mesh in the config,
a double and 3 track mesh is exported into trainz as 2 or 3 I believe.
someone can correct me if I'm wrong.
-Aaron