As I understand it there is a tool in trainz itself that can create the .texture files from Targa.
I don`t have trainz myself and I added the .texture reading at the request of a trainz user who sent me some example files. He said he just needed a tool to convert the files to an editable format as the trainz tools can do the job of converting back.
This was a couple of years ago and I think there have been several new versions of Trainz since then