Trainz is likely trying to rebuild it's assets.tdx file. Navigate to your TRS installation directory, look for a folder named "cache", and see if there is a file inside called "assets.bku"
COPY the file to the main directory and rename it to "assets.tdx". Hopefully the file is there. If not, you may have to let Trainz sit while it rebuilds the file.
Cheers,
John