.texture files are common in older Trainz assets. The latest Trainz version will use normal graphic files (.jpg, .tga, .png, etc) and .texture.txt files (normal graphic files listed in a text file). .texture files are currently mostly found in Built-in and Base assets that are installed with Trainz and are compressed to save space and are faster to load.
I have just one Payware texture asset, <kuid:605579:100054> Asphalt, out of a total of 2484 texture assets installed. This asset is listed as "Payware, Packaged" which means that it is encrypted and cannot be modified or copied. That particular asset contains a number of .texture files. So, it seems that the answer to your question is overwhelmingly (but not totally) NO.
Some further figures.
Of those 2484 installed texture assets, 1847 are "Packaged" which means that they are part of an installed DLC package. Only 21 of those "Packaged" texture assets are not on the DLS and are only found in payware DLC. The remainder are on the DLS and are "Freeware", not "Payware".
Of those 21 "Packaged" and only in "Payware DLC" texture assets, the majority (of those I checked) had .texture files.
So a more accurate answer is that the vast majority of texture assets containing .texture files are not Payware.
I hope this is what you were looking for.
