If you have not already done this then open the asset in Explorer using CM. Look through its files for any of type .tga .png .jpg .bmp .jpg or .jpeg (very rare to find other common image formats). The "paintwork" will probably be one of them.
If it is an older or a built-in (or base asset) then it may have the file type .texture
.texture files in older/base/built-in assets use a compressed image format which you will have to convert to one of the above (.texture files are not to be confused with the .texture.txt files, a completely different format, now used in modern assets).
PEVsoft utilities (you will need to Google them as I don't have them) are free utilities designed specifically for Trainz to perform the conversion from .texture toanother image format.
Another possibility is that the signal may simply be an alias of another signal asset. Its config.txt file will reference the kuid of the source asset and use its image files.
