If im right in thinking, all you need to do is do a search for the file, once you've found it right-click, copy and then paste it into the file of the loco. then commit it and should work fine. Im pretty sure thast how you do it, but there is so many errors out there, its easy to get confused :hehe:
Edit LUL 1967 tube stock A (Refurb). Then open the folder lu_1967_front_art. Then edit the file lu_1967_front_art_icon.texture. Add primary= to the first line. Save and close all files, then commit.