Which version of Trainz is this in? If its TANE then expect to see alot of that same error. Easy fix is to open the asset in Explorer, then open each .texture in PEVsoft's Images2TGA, then click "Save As", don't change the name or anything else, and save it to the same file you found it in. Once you've done this for all the textures, close everything down and Submit Edits or Commit Asset, and you're done.
If its TS12, I would think the above method would still work, but not as sure.
Falcus